js: Remove unnecessary versions of clutter_actor_get_children

clutter_actor_get_children requires making a temporary GSList from
a linked list structure, and then creating a JS Array from that GSList.
For simple cases like the number of children, use clutter_actor_get_n_children.

https://bugzilla.gnome.org/show_bug.cgi?id=677426

js/ui/popupMenu.js

@@ -906,7 +906,7 @@ const PopupMenuBase = new Lang.Class({
     },
 
     isEmpty: function() {
-        return this.box.get_children().length == 0;
+        return this.box.get_n_children() == 0;
     },
 
     isChildMenu: function(menu) {