Rework Playlist and Playback Controller to submit/react to events

[wrichter]

depends on #15

Instead of the Playback Controller polling the Playlist for the first track, the Playlist should submit an event what the first track is which the Playback controller reacts upon. This would make it easier to handle multiple playbacks in parallel (because the playback controller needs no awareness which playlists exist/are active, it just reacts on events.