Improve GUI of NuGet for Unity window (#617)

* refresh the look of the editor window and bring it in line with the modern Unity editor
* Updated images in docs
* Cached GUI styles
* Improve code quality of NugetWindow.cs

---------

Co-authored-by: JoC0de <[email protected]>

src/NuGetForUnity/Editor/Ui/RectHelper.cs

@@ -0,0 +1,99 @@
+using UnityEngine;
+
+namespace NugetForUnity
+{
+    /// <summary>
+    ///     Extension methods for manipulating Rect objects.
+    /// </summary>
+    internal static class RectHelper
+    {
+        /// <summary>
+        ///     Expands the boundaries of the Rect by a specified value in all directions.
+        /// </summary>
+        /// <param name="rect">The rect to update.</param>
+        /// <param name="value">The value to add.</param>
+        /// <returns>The updated Rect.</returns>
+        public static Rect Expand(this Rect rect, float value)
+        {
+            rect.xMin -= value;
+            rect.xMax += value;
+            rect.yMin -= value;
+            rect.yMax += value;
+            return rect;
+        }
+
+        /// <summary>
+        ///     Expands the Rect horizontally by a specified value.
+        /// </summary>
+        /// <param name="rect">The rect to update.</param>
+        /// <param name="value">The value to add.</param>
+        /// <returns>The updated Rect.</returns>
+        public static Rect ExpandX(this Rect rect, float value)
+        {
+            rect.xMin -= value;
+            rect.xMax += value;
+            return rect;
+        }
+
+        /// <summary>
+        ///     Expands the Rect vertically by a specified value.
+        /// </summary>
+        /// <param name="rect">The rect to update.</param>
+        /// <param name="value">The value to add.</param>
+        /// <returns>The updated Rect.</returns>
+        public static Rect ExpandY(this Rect rect, float value)
+        {
+            rect.yMin -= value;
+            rect.yMax += value;
+            return rect;
+        }
+
+        /// <summary>
+        ///     Adds a specified value to the X-coordinate of the Rect.
+        /// </summary>
+        /// <param name="rect">The rect to update.</param>
+        /// <param name="value">The value to add.</param>
+        /// <returns>The updated Rect.</returns>
+        public static Rect AddX(this Rect rect, float value)
+        {
+            rect.x += value;
+            return rect;
+        }
+
+        /// <summary>
+        ///     Adds a specified value to the Y-coordinate of the Rect.
+        /// </summary>
+        /// <param name="rect">The rect to update.</param>
+        /// <param name="value">The value to add.</param>
+        /// <returns>The updated Rect.</returns>
+        public static Rect AddY(this Rect rect, float value)
+        {
+            rect.y += value;
+            return rect;
+        }
+
+        /// <summary>
+        ///     Sets the width of the Rect to a specified value.
+        /// </summary>
+        /// <param name="rect">The rect to update.</param>
+        /// <param name="value">The value to set.</param>
+        /// <returns>The updated Rect.</returns>
+        public static Rect SetWidth(this Rect rect, float value)
+        {
+            rect.width = value;
+            return rect;
+        }
+
+        /// <summary>
+        ///     Sets the height of the Rect to a specified value.
+        /// </summary>
+        /// <param name="rect">The rect to update.</param>
+        /// <param name="value">The value to set.</param>
+        /// <returns>The updated Rect.</returns>
+        public static Rect SetHeight(this Rect rect, float value)
+        {
+            rect.height = value;
+            return rect;
+        }
+    }
+}