[notifications] Introduce a list of apps that are limited to single notification only

This prevents some apps to spam lots of notifications when all it needs
is actually just one single notification.

Imagine you're switching songs in your playlist quickly and each song
change sends new notification, but when you get to the tenth song, you
still see the notification from the 3rd song (because timeouts) and you
don't really care about all those songs changes still in the queue as
you see it in the playlist anyway.

So this patch limits certain apps to have only one single notification
which is always updated. So far Clementine and Spotify is there.
Switching songs or changing playback status creates only one single
popup.

Additionally, the list gets added entries from a config file too.

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