xdndHandler: prevent dummy actor from interfering with Hot Corner

Since the dummy actor occupies exactly the same area of the Hot Corner,
it can be erroneously picked during xdnd operations. Fix this by
hiding it from pick.

https://bugzilla.gnome.org/show_bug.cgi?id=669831

js/ui/xdndHandler.js

@@ -17,6 +17,7 @@ const XdndHandler = new Lang.Class({
         // 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 });
         global.stage.add_actor(this._dummy);
+        Shell.util_set_hidden_from_pick(this._dummy, true);
         this._dummy.hide();
 
         // Mutter delays the creation of the output window as long