popupMenu: Remove our custom allocation code

With support for column-based layout gone, simply use a box layout
and allow items to use their own layouts without any "framework".

https://bugzilla.gnome.org/show_bug.cgi?id=705845

js/ui/remoteMenu.js

@@ -120,7 +120,7 @@ const RemoteMenuItemMapper = new Lang.Class({
 
         this.menuItem = new PopupMenu.PopupBaseMenuItem();
         this._label = new St.Label();
-        this.menuItem.addActor(this._label);
+        this.menuItem.actor.add_child(this._label);
         this.menuItem.actor.label_actor = this._label;
 
         this.menuItem.connect('activate', Lang.bind(this, function() {