message: camera: notify after subscription
  It would be handy to get an update right after subscribing.
diff: @@ -1238,12 +1238,14 @@ void CameraImpl::subscribe_current_settings(
  const Camera::subscribe_current_settings_callback_t &callback)
  {
  _subscribe_current_settings_callback = callback;
  + notify_current_settings();
  }
  void CameraImpl::subscribe_possible_setting_options(
  const Camera::subscribe_possible_setting_options_callback_t &callback)
  {
  _subscribe_possible_setting_options_callback = callback;
  + notify_possible_setting_options();
  }
  void CameraImpl::notify_current_settings()
  