Made RadioGroup utilize InvertColorsOnPress.

Works, but not optimally. The entire view invert. What would be really nice is if just the radio item under the mouse inverted. We can fix this later.

Terminal.Gui/Views/RadioGroup.cs

@@ -76,6 +76,8 @@ public class RadioGroup : View
         KeyBindings.Add (Key.Space, Command.Accept);
 
         LayoutStarted += RadioGroup_LayoutStarted;
+
+        InvertColorsOnPress = true;
     }
 
     /// <summary>
@@ -163,6 +165,7 @@ public class RadioGroup : View
     /// <inheritdoc/>
     protected internal override bool OnMouseEvent  (MouseEvent me)
     {
+        base.OnMouseEvent (me);
         if (!me.Flags.HasFlag (MouseFlags.Button1Clicked))
         {
             return false;