Add D-Bus annotations support to methods, signals and properties.

* mandos (dbus_annotations): New; decorator to add annotations.
  (DBusObjectWithProperties._get_all_dbus_things): Bug fix: return
                                                   normal name if no
                                                   "_dbus_name"
                                                   attribute.
  (DBusObjectWithProperties.Introspect): Add annotation tags.
  (AlternateDBusNamesMetaclass.__new__): Copy annotations.