message: Rename type alias to `NotificationClickedSignal`
diff: @@ -39,7 +39,7 @@ public:
  NotificationType type{NotificationType::Information};
  };
  - using NotificationCallback = NAS2D::Signal<const Notification&>;
  + using NotificationClickedSignal = NAS2D::Signal<const Notification&>;
  public:
  NotificationArea();
  @@ -48,7 +48,7 @@ public:
  void push(Notification notification);
  void clear();
  - NotificationCallback::Source& notificationClicked() { return mNotificationClicked; }
  + NotificationClickedSignal::Source& notificationClicked() { return mNotificationClicked; }
  void update() override;
  @@ -65,7 +65,7 @@ private:
  std::vector<Notification> mNotificationList;
  std::size_t mNotificationIndex;
  - NotificationCallback mNotificationClicked;
  + NotificationClickedSignal mNotificationClicked;
  };
  void drawNotificationIcon(NAS2D::Point<int> position, NotificationArea::NotificationType type, const NAS2D::Image& icons);
  