Don't override UserControl.StyleKey.

Instead of pinning `UserControl.StyleKey` at `typeof(UserControl)`, make the `UserControl` default template selector `:is(UserControl)`. This will allow controls derived from `UserControl` to be more easily styled.

src/Avalonia.Controls/UserControl.cs

@@ -27,9 +27,6 @@ namespace Avalonia.Controls
             remove { _nameScope.Unregistered -= value; }
         }
 
-        /// <inheritdoc/>
-        Type IStyleable.StyleKey => typeof(UserControl);
-
         /// <inheritdoc/>
         void INameScope.Register(string name, object element)
         {

src/Avalonia.Themes.Default/UserControl.xaml

@@ -1,4 +1,4 @@
-<Style xmlns="https://github.com/avaloniaui" Selector="UserControl">
+<Style xmlns="https://github.com/avaloniaui" Selector=":is(UserControl)">
   <Setter Property="Template">
     <ControlTemplate>
       <ContentPresenter Name="PART_ContentPresenter"