add Tree::add_o_ref
add Tree::add_o_ref

This lets us add an ObjectPath to a Tree we only have a &mut for.

Also alters remove() return type for consistency with other APIs.

Signed-off-by: Andy Grover <b7d524d2f5cc5aebadb6b92b08d3ab26911cde33@redhat.com>

manuels/dbus-rs