remoteMenu: Add actionGroup accessor

To avoid recreating the app menu unnecessarily, the panel checks
whether the menu's current actionGroup already matches the target
one. However as the menu's actionGroup property is currently private,
the test always fails, whoops.

https://bugzilla.gnome.org/show_bug.cgi?id=781471

js/ui/remoteMenu.js

@@ -192,6 +192,10 @@ const RemoteMenu = new Lang.Class({
                                               _removeItem.bind(null, this));
     },
 
+    get actionGroup() {
+        return this._actionGroup;
+    },
+
     destroy: function() {
         this._tracker.destroy();
         this.parent();