st-container: Remove st_container_get_children_list

Replace it with the new actor iteration APIs. This fixes a few
unintentional memory leaks - st_container_get_children_list
returns an internal list, and clutter_actor_get_children_list
allocates a new list.

https://bugzilla.gnome.org/show_bug.cgi?id=670034

https://bugzilla.gnome.org/show_bug.cgi?id=670910