- Added _category_filter_pick_elements() function, to remove distant-parent

'update' form buttons before calling category_node_save(), or before calling
taxonomy_node_save() for the wrapper.
- Updated category_del_container() to remove container IDs from
'category_allowed_containers_[nodetype]' variables on delete.
- Fixed incorrect logic in category_check_container_hierarchy().
- Changed the parent select form element, so that it doesn't appear for
free-tagging categories, and the user is instead given a message saying that
free-tagging categories always have their container set as their parent.
- Fixed a bug in wrapper's taxonomy_node_save() function - assigned category
objects not getting converted to terms.