Initialize DevTools MainWindow theme in code.

Previously if no theme was included in App.xaml, then DevTools would fail to open because the `StaticResource` lookup would fail due to the window's XAML not being parsed yet.

src/Avalonia.Diagnostics/Diagnostics/Views/MainWindow.xaml

@@ -3,8 +3,7 @@
         xmlns:views="clr-namespace:Avalonia.Diagnostics.Views"
         xmlns:diag="clr-namespace:Avalonia.Diagnostics"
         Title="Avalonia DevTools"
-        x:Class="Avalonia.Diagnostics.Views.MainWindow"
-        Theme="{StaticResource {x:Type Window}}">
+        x:Class="Avalonia.Diagnostics.Views.MainWindow">
   <Window.DataTemplates>
     <diag:ViewLocator/>
   </Window.DataTemplates>

src/Avalonia.Diagnostics/Diagnostics/Views/MainWindow.xaml.cs

@@ -28,6 +28,11 @@ namespace Avalonia.Diagnostics.Views
         {
             InitializeComponent();
 
+            // Apply the SimpleTheme.Window theme; this must be done after the XAML is parsed as
+            // the theme is included in the MainWindow's XAML.
+            if (Theme is null && this.FindResource(typeof(Window)) is ControlTheme windowTheme)
+                Theme = windowTheme;
+
             _keySubscription = InputManager.Instance?.Process
                 .OfType<RawKeyEventArgs>()
                 .Where(x => x.Type == RawKeyEventType.KeyDown)