Fixed implementation to also respond to introspection on partial object paths

Using d-feet, my txdbus DBus services were not introspectable, leading to an 'invalid object path' error. These 2 commits fixes this by delivering the expected available paths from a introspected partial path request.

Say your DBus ObjectPath is "/a/b/c". d-feet then requests initially an introspection on path /, which lead to an error before. Now it just returns "<node name="a" />" on this request, which leads to further requests from d-feet.