Delete notifications upon closing the app

[Ingvix]

Usually when I close the app it's when I'm back at my PC but notice getting a notification on the app. It's a bit annoying to go manually clear the notifications separately before or after closing the app so I'd find it better for them to get deleted upon closing the app.

In case some people like the current behaviour, it could perhaps be introduced as a setting?

[Wunderfitz]

Is probably an easy one, remove existing notifications here: https://github.com/Wunderfitz/harbour-fernschreiber/blob/master/src/notificationmanager.cpp#L167

[monich]

And yes, it would have to be configurable. Some people do like the current behavior.