Playlists should not be tied to a music source. For
example, I should be able to enqueue songs from the Rio
Server and a Shoutcast stream into the same playlist.
This will be valuable if we support other server types
in the future. Also, I should be able to add music to
the queue without creating a new playlist. I.E. select
an album to play, then come back later and add another
album to the playlist without replacing the first.