Add keyboard shortcut support to ButtonWidgets.

Hook up 'escape' and 'return' as appropriate for moving through menus.

OpenRA.Game/Widgets/Widget.cs

@@ -245,7 +245,7 @@ namespace OpenRA.Widgets
                 return false;
 
             // Can any of our children handle this?
-            foreach (var child in Children)
+			foreach (var child in Children.OfType<Widget>().Reverse())
                 if (child.HandleKeyPressOuter(e))
                     return true;