xdndHandler: Ensure that the XDnD clone is on top of the panel

The panel is added to the uiGroup, not the overlay group, so
to stack on top of it, we need to add ourselves to the uiGroup
as well.

https://bugzilla.gnome.org/show_bug.cgi?id=695073

js/ui/xdndHandler.js

@@ -68,7 +68,7 @@ const XdndHandler = new Lang.Class({
                                                                    source: cursorWindow});
 
             this._cursorWindowClone = new Clutter.Clone({ source: cursorWindow });
-            global.overlay_group.add_actor(this._cursorWindowClone);
+            Main.uiGroup.add_actor(this._cursorWindowClone);
             Shell.util_set_hidden_from_pick(this._cursorWindowClone, true);
 
             // Make sure that the clone has the same position as the source