Add "Play next episode automatically" setting to subscriptions.
Simplify how podcasts are being played.

And add first code to start next episode playback automatically.
First implementation of playlist support.

We serialize the playlist information from the foreground application to the audio agent. The audio agent can then contruct its playlist and play the next episode when a track ends.

Todo:
- Update player UI when a new track starts to play.
- Play history.