citra_qt/multiplayer: Add user ping support

The user would be notified if the message contains "@" followed by the user's nickname or forum username. An alert would be shown, and the icon and message in the status bar would be changed. All notification is only shown if the chat window currently does not have focus.

Also added a connected_notification icon for showing in the status bar.

src/citra_qt/multiplayer/client_room.h

@@ -27,6 +27,7 @@ public slots:
 signals:
     void RoomInformationChanged(const Network::RoomInformation&);
     void StateChanged(const Network::RoomMember::State&);
+    void ShowNotification();
 
 private:
     void Disconnect();