Bug 707255 - Open preferences when quitting causes assertion failure

When removing the container of a GimpContainerView,
gimp_container_view_remove_container() must be the last call. It was
causing a `GIMP_IS_CONTAINER (container)' failure in subsequent
gimp_container_get_children_type().
For good practice, unsetting a container works now the exact reversed
order as the setting of a container.