popupMenu: Complete the dummy fake menu

"toggle" is used when clicking on the source actor. As we don't
actually have a menu, we don't want to actually take any grabs
through the menu system, so make it a no-op rather than have it
toggle the menu state.

https://bugzilla.gnome.org/show_bug.cgi?id=689954

js/ui/popupMenu.js

@@ -1289,6 +1289,7 @@ const PopupDummyMenu = new Lang.Class({
 
     open: function() { this.emit('open-state-changed', true); },
     close: function() { this.emit('open-state-changed', false); },
+    toggle: function() {},
     destroy: function() {
         this.emit('destroy');
     },