A tree-enabled replacement for gimp_container_add,remove_handler(),
slightly more sane to a degree that makes me think it could replace
GimpContainer's own handler stuff. Public API consists of two
function:
- gimp_tree_handler_connect() which connects recursively to a specific
signal of all items in a container tree.
- gimp_tree_handler_disconnect() is just a wrapper around unrefing the
handler, but makes sure that dispose() is really executed (which
actually disconnects things).