Refactor D-Bus annotation class out from D-Bus properties class.

* mandos (DBusObjectWithAnnotations): New; factored out D-Bus
                                      annotations code from
                                      DBusObjectWithProperties class.
  (DBusObjectWithProperties): Inherit from DBusObjectWithAnnotations.
  (main/MandosDBusService): - '' -
  (main/MandosDBusService._foo): Removed interface annotation for
                                 "o.f.D.Property.EmitsChangedSignal";
                                 this object has no properties.