MVC: Music 'view' and controller

Music playing could be refactored into:
* gameplay events for starting music
* ui events for stopping and starting music
* controller that keeps track of the current music, the list of music data, and sends ui events to start and stopping individual musics
* "view" implementations of music that play; implement their own fade routines

Stopping and starting music ui events would include how fast they should fade in or out.

Map the current music-playing implementation before deciding how gameplay events, UI events, the music controller, and individual music views should interact. Define the event fade parameters and the boundaries for tracking current music and music data; done means the proposed responsibilities are implemented and music transitions still work as intended.

Written by the indexing model from the issue text.