Better tab controls (#336)

This is a continuation of the bare-bones tooling added by #332; the
layout and rendering of a tab control is now encapsulated into a
`TabControl` class. That class takes care of rendering all the tabs and
drawing only the correct tab content, as long as you give it string for
each title and a `GuiBuilder` to render each tab body.

For testing, I declared various sets of tabs, changed the `TabControl`
properties, and made sure the tabs rendered and reacted as expected.
Eventually I ended up with this monstrosity:
```
    private readonly TabControl tabControl = new(("General", DrawGeneral), ("Progression", DrawProgression), ("Tab 1", null), ("Tab 2", null), ("Tab 3", null), ("Tab 4", null), ("Tab 5's title text causes fewer problems", null), ("Tab 6", null), ("Tab 7", null), ("Tab 8 has an obscenely long title text for the purpose of causing as many problems as physically possible", null), ("Tab 9 wants three lines\nin its text, but\nthat doesn't work", null));
```
I expected Tab 9's title to render on three lines of text, but it only
renders on one line. I decided to leave multi-line tab titles as an
project for another time.

Yafc.UI/ImGui/ImGuiLayout.cs

@@ -267,6 +267,7 @@ public sealed class OverlappingAllocations : IDisposable {
         private readonly ImGui gui;
         private readonly bool initialDrawState;
         private readonly float initialTop;
+        private float maximumBottom;
 
         internal OverlappingAllocations(ImGui gui, bool alsoDraw) {
             this.gui = gui;
@@ -282,10 +283,14 @@ internal OverlappingAllocations(ImGui gui, bool alsoDraw) {
         /// If <see langword="false"/>, subsequent drawing commands (for the next tab page) will allocate space, but will not draw their controls.</param>
         public void StartNextAllocatePass(bool alsoDraw) {
             gui.enableDrawing = initialDrawState && alsoDraw;
+            maximumBottom = Math.Max(gui.state.top, maximumBottom);
             gui.state.top = initialTop;
         }
 
-        public void Dispose() => gui.enableDrawing = initialDrawState;
+        public void Dispose() {
+            gui.enableDrawing = initialDrawState;
+            gui.state.top = maximumBottom;
+        }
     }
 
     public void SetMinWidth(float width) {