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/status/keyboard.js

@@ -203,8 +203,8 @@ const LayoutMenuItem = new Lang.Class({
 
         this.label = new St.Label({ text: displayName });
         this.indicator = new St.Label({ text: shortName });
-        this.addActor(this.label);
-        this.addActor(this.indicator);
+        this.actor.add(this.label, { expand: true });
+        this.actor.add(this.indicator);
         this.actor.label_actor = this.label;
     }
 });