listen to the favorites of a user

hi, again! :)

usually i listen to the favs from my friends. i really needed that feature, maybe you like it too and it can be merged.

this feature is bind to the 'f' key.
if you never navigated to a user before it asks for the username otherwise it uses the last known user. i'm not sure about this behavior but it worked best for me. i tried asking for the username every time but it got on my nerves after a short time.

i had to change some of the internals in TrackCollection, at the moment TrackCollection can only handle the user tracks and the most recent tracks (without scope) this introduces a new state inside TrackCollection called 'collection_to_load'. maybe its better not to reuse the TrackCollection but since you did it with the user and recent tracks i followed the schema.

have a good night
ben