Implement queue

[moritz-weber]

• implement actions to set/retrieve queue from background task
• implement UI to display queue
• implement logic to play next song in queue after current one finished

BUGS

• NextIndicator and Previous/NextButton are not updating, when CurrentlyPlaying wasn't focused

[moritz-weber]

• background task needs access to database -> MoorIsolate
• bg task gets spawned by AudioManager
• plugging MoorIsolate directly into AudioManager seems like a bad idea...

[moritz-weber]

• Problem: MoorMusicDataSource is not initialized in background task, when first called --> ask on audio_service GitHub or create a custom event that the foreground task awaits

Idea:

• do nothing in onStart (only completer.future)
• after calling start, send a custom action to do the initializing and wait for that to finish

or

• don't initialize MoorMusicDataSource on Start; instead check for null on usage and initialize when needed (maybe with a getter)