Combined UI for both multizone stream state and playlist management

[markferry]

There is no good UI for both multizone stream state and playlist management.

Options:

1. Spotify/Airplay/MPD multizone volume APIs?
2. HA media browsing for MPD?
3. Iris group configuration UI (pop-up?)

Iris shows snapcast group volumes ("zone balance" in HA), but group members are buried in settings.

Switching to user-controllable snapcast volume in the front-end means finding a way to have Spotify, mpd and Airplay control snapcast group volume.