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).