MVC: Music 'view' and controller

[evktalo]

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.

[evktalo]

Nice step forwards in 203d27e

[evktalo]

Probably some sort of a view model is what I'm thinking of here ...