Allow child paths to be introspectable
Allow child paths to be introspectable

Add `<node name="[child_path]"/>` to the introspection XML for all
potential child paths that have already been defined. This makes it
possible to find the children of a path using tools like `busctl tree`.

Fixes #149

Shouqun/node-dbus,Shouqun/node-dbus,Shouqun/node-dbus