Listenbrainz submission

Enables submission of listens (scrobbles) and now playing notifications to ListenBrainz. Fixes #1342 (closed)

To enable it you have to set your personal user token from your ListenBrainz profile page in settings:

This is now mostly finished. The only thing missing would be proxy support, I saw the last.fm implementation supports this, I think the ListenBrainz implementation should support this as well.

@gnumdk Btw, really great project. I like the player very much, and the code base is very clean and pretty easy to get into :)