popupMenu: Raise menu when popping it up

When a menu gets popped up, it should never
pop behind anything else in the shell.

https://bugzilla.gnome.org/show_bug.cgi?id=657082

js/ui/popupMenu.js

@@ -1112,6 +1112,8 @@ PopupMenu.prototype = {
         this._boxPointer.setPosition(this.sourceActor, this._alignment);
         this._boxPointer.show(animate);
 
+        this.actor.raise_top();
+
         this.emit('open-state-changed', true);
     },