Don't special case on view_iface->model_is_tree and always run
gimp_container_view_remove_foreach(), also on the view's toplevel
container.  Run gimp_container_view_clear_items() anyway on the
toplevel as an optimization, but with a big comment. This makes all
views (on list *and* tree models) behave the same way, and makes
view_iface->model_is_tree practically obsolete, will remove it later.