* Once Vala gets Immutable -support, check that ServiceIntrospectionCallback 
   and methods in GUPnPServiceIntrospection work without warnings
   (see tests/introspection-test for examples) 

 * At the moment it does not seem possible to define signal argument type and 
   contained types in metadata, so that's all done in custom.vala. Example:
       public signal void resource_available (string usn, 
                                              GLib.List<string> locations);
   Currently that "GLib.List<string>" is done in custom.vala. 
   This will probably be possible in metadata in the future.