Remove st_container_remove_all & rewrite st_container_destroy_children

Remove st_container_remove_all & rewrite st_container_destroy_children

1. Both functions leaked the nodes in priv->children
2. st_container_remove_all wasn't properly updating first_child and last_child
3. remove_all() is almost never right since it won't cause signal handlers
   on the children to be removed. In the rare cases where it might be needed
   the caller can simply use clutter_container_remove().
https://bugzilla.gnome.org/show_bug.cgi?id=640781