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

Review Request #118796 - Created June 17, 2014 and submitted - Latest diff uploaded

Martin Klapetek
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.