box: port a ClutterGroup::foreach fix to ClutterBox

box: port a ClutterGroup::foreach fix to ClutterBox

ClutterGroup::foreach was recently changed (ref: ce030a3fce) to use
g_list_foreach() to iterate the children instead of manually iterating
the list so it would safely handle calls like:

  clutter_container_foreach (container, clutter_actor_destroy);

  (In this example clutter_actor_destroy will result in the current
   list item being iterated being freed.)