First part

FilterExtension.sln

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+
+Microsoft Visual Studio Solution File, Format Version 12.00
+# Visual Studio 2013
+VisualStudioVersion = 12.0.30501.0
+MinimumVisualStudioVersion = 10.0.40219.1
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "PartFilters", "FilterExtension\PartFilters.csproj", "{8633A568-654E-409D-A7B2-95839F2EBD3D}"
+EndProject
+Global
+	GlobalSection(SolutionConfigurationPlatforms) = preSolution
+		Debug|Any CPU = Debug|Any CPU
+		Release|Any CPU = Release|Any CPU
+	EndGlobalSection
+	GlobalSection(ProjectConfigurationPlatforms) = postSolution
+		{8633A568-654E-409D-A7B2-95839F2EBD3D}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{8633A568-654E-409D-A7B2-95839F2EBD3D}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{8633A568-654E-409D-A7B2-95839F2EBD3D}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{8633A568-654E-409D-A7B2-95839F2EBD3D}.Release|Any CPU.Build.0 = Release|Any CPU
+	EndGlobalSection
+	GlobalSection(SolutionProperties) = preSolution
+		HideSolutionNode = FALSE
+	EndGlobalSection
+EndGlobal

FilterExtension/ControlSurface.cs

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+using System;
+using System.Collections.Generic;
+using System.Linq;
+using UnityEngine;
+
+namespace PartFilters
+{
+    class ControlSurface
+    {
+        internal void SurfaceFilter()
+        {
+            PartCategorizer.Icon icon = PartCategorizer.Instance.GetIcon("R&D_node_icon_stability");
+
+            PartCategorizer.Category filterByFunction = PartCategorizer.Instance.filters.Find(f => f.button.categoryName == "Filter by Function");
+            PartCategorizer.AddCustomSubcategoryFilter(filterByFunction, "Control Surfaces", icon, p => p.moduleInfos.Any(m => m.moduleName == "Control Surface" || m.moduleName == "FAR Controllable"));
+
+            RUIToggleButtonTyped button = filterByFunction.button.activeButton;
+            button.SetFalse(button, RUIToggleButtonTyped.ClickType.FORCED);
+            button.SetTrue(button, RUIToggleButtonTyped.ClickType.FORCED);
+        }
+    }
+}

FilterExtension/Core.cs

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+using System;
+using System.Collections.Generic;
+using UnityEngine;
+
+namespace PartFilters
+{
+    [KSPAddon(KSPAddon.Startup.MainMenu, true)]
+    public class Core : MonoBehaviour
+    {
+        Wheel wheelFilt;
+        ControlSurface contrSurfaceFilt;
+
+        void Awake()
+        {
+            GameEvents.onGUIEditorToolbarReady.Add(Filters);
+
+            wheelFilt = new Wheel();
+            contrSurfaceFilt = new ControlSurface();
+        }
+
+        private void Filters()
+        {
+            wheelFilt.WheelFilter();
+            contrSurfaceFilt.SurfaceFilter();
+        }
+    }
+}

FilterExtension/PartFilters.csproj

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+<?xml version="1.0" encoding="utf-8"?>
+<Project ToolsVersion="12.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+  <Import Project="$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props" Condition="Exists('$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props')" />
+  <PropertyGroup>
+    <Configuration Condition=" '$(Configuration)' == '' ">Debug</Configuration>
+    <Platform Condition=" '$(Platform)' == '' ">AnyCPU</Platform>
+    <ProjectGuid>{8633A568-654E-409D-A7B2-95839F2EBD3D}</ProjectGuid>
+    <OutputType>Library</OutputType>
+    <AppDesignerFolder>Properties</AppDesignerFolder>
+    <RootNamespace>PartFilters</RootNamespace>
+    <AssemblyName>PartFilters</AssemblyName>
+    <TargetFrameworkVersion>v3.5</TargetFrameworkVersion>
+    <FileAlignment>512</FileAlignment>
+    <TargetFrameworkProfile />
+  </PropertyGroup>
+  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' ">
+    <DebugSymbols>true</DebugSymbols>
+    <DebugType>full</DebugType>
+    <Optimize>false</Optimize>
+    <OutputPath>..\..\..\..\..\Desktop\Kerbal Space Program Dev\GameData\Filters\</OutputPath>
+    <DefineConstants>DEBUG;TRACE</DefineConstants>
+    <ErrorReport>prompt</ErrorReport>
+    <WarningLevel>4</WarningLevel>
+  </PropertyGroup>
+  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Release|AnyCPU' ">
+    <DebugType>pdbonly</DebugType>
+    <Optimize>true</Optimize>
+    <OutputPath>bin\Release\</OutputPath>
+    <DefineConstants>TRACE</DefineConstants>
+    <ErrorReport>prompt</ErrorReport>
+    <WarningLevel>4</WarningLevel>
+  </PropertyGroup>
+  <ItemGroup>
+    <Reference Include="Assembly-CSharp">
+      <HintPath>..\..\..\Desktop\Kerbal Space Program Dev\KSP_Data\Managed\Assembly-CSharp.dll</HintPath>
+    </Reference>
+    <Reference Include="System" />
+    <Reference Include="System.Core" />
+    <Reference Include="System.Xml.Linq" />
+    <Reference Include="System.Data.DataSetExtensions" />
+    <Reference Include="System.Data" />
+    <Reference Include="System.Xml" />
+    <Reference Include="UnityEngine">
+      <HintPath>..\..\..\Desktop\Kerbal Space Program Dev\KSP_Data\Managed\UnityEngine.dll</HintPath>
+    </Reference>
+  </ItemGroup>
+  <ItemGroup>
+    <Compile Include="ControlSurface.cs" />
+    <Compile Include="Core.cs" />
+    <Compile Include="Properties\AssemblyInfo.cs" />
+    <Compile Include="Wheel.cs" />
+  </ItemGroup>
+  <Import Project="$(MSBuildToolsPath)\Microsoft.CSharp.targets" />
+  <!-- To modify your build process, add your task inside one of the targets below and uncomment it. 
+       Other similar extension points exist, see Microsoft.Common.targets.
+  <Target Name="BeforeBuild">
+  </Target>
+  <Target Name="AfterBuild">
+  </Target>
+  -->
+</Project>

FilterExtension/Properties/AssemblyInfo.cs

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+using System.Reflection;
+using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
+
+// General Information about an assembly is controlled through the following 
+// set of attributes. Change these attribute values to modify the information
+// associated with an assembly.
+[assembly: AssemblyTitle("PartFilters")]
+[assembly: AssemblyDescription("")]
+[assembly: AssemblyConfiguration("")]
+[assembly: AssemblyCompany("")]
+[assembly: AssemblyProduct("PartFilters")]
+[assembly: AssemblyCopyright("Copyright ©  2014")]
+[assembly: AssemblyTrademark("")]
+[assembly: AssemblyCulture("")]
+
+// Setting ComVisible to false makes the types in this assembly not visible 
+// to COM components.  If you need to access a type in this assembly from 
+// COM, set the ComVisible attribute to true on that type.
+[assembly: ComVisible(false)]
+
+// The following GUID is for the ID of the typelib if this project is exposed to COM
+[assembly: Guid("7397ad0e-7874-458b-9b25-d904a6c98b4a")]
+
+// Version information for an assembly consists of the following four values:
+//
+//      Major Version
+//      Minor Version 
+//      Build Number
+//      Revision
+//
+// You can specify all the values or you can default the Build and Revision Numbers 
+// by using the '*' as shown below:
+// [assembly: AssemblyVersion("1.0.*")]
+[assembly: AssemblyVersion("1.0.0.0")]
+[assembly: AssemblyFileVersion("1.0.0.0")]

FilterExtension/Wheel.cs

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+using System;
+using System.Collections.Generic;
+using System.Linq;
+using UnityEngine;
+
+namespace PartFilters
+{
+    class Wheel
+    {
+        internal void WheelFilter()
+        {
+            PartCategorizer.Icon icon = PartCategorizer.Instance.GetIcon("R&D_node_icon_fieldscience");
+
+            PartCategorizer.Category filterByFunction = PartCategorizer.Instance.filters.Find(f => f.button.categoryName == "Filter by Function");
+            PartCategorizer.AddCustomSubcategoryFilter(filterByFunction, "Wheels", icon, p => p.moduleInfos.Any(m => m.moduleName == "Wheel" || m.moduleName == "Landing Gear"));
+
+            RUIToggleButtonTyped button = filterByFunction.button.activeButton;
+            button.SetFalse(button, RUIToggleButtonTyped.ClickType.FORCED);
+            button.SetTrue(button, RUIToggleButtonTyped.ClickType.FORCED);
+        }
+    }
+}