Remove simple uses of ClutterRectangle

https://bugzilla.gnome.org/show_bug.cgi?id=699975

js/ui/xdndHandler.js

@@ -16,7 +16,7 @@ const XdndHandler = new Lang.Class({
         this._cursorWindowClone = null;
 
         // Used as a drag actor in case we don't have a cursor window clone
-        this._dummy = new Clutter.Rectangle({ width: 1, height: 1, opacity: 0 });
+        this._dummy = new Clutter.Actor({ width: 1, height: 1, opacity: 0 });
         Main.uiGroup.add_actor(this._dummy);
         Shell.util_set_hidden_from_pick(this._dummy, true);
         this._dummy.hide();