Use prettier notification images

Avatars are shown rounded in the main interface, so they should look the
same in the notification tray too. On top of that, if the room is a
group then show that group's icon when applicable in the bottom right.

src/notificationsmanager.h

@@ -97,4 +97,7 @@ private:
 
 private Q_SLOTS:
     void processNotificationJob(QPointer<Quotient::Connection> connection, Quotient::GetNotificationsJob *job, bool initialization);
+
+private:
+    QPixmap createNotificationImage(const QImage &icon, NeoChatRoom *room);
 };