Only unpause music after playing game if it was automatically paused

[jerith]

This implements the non-persistent half of #34, because having the music turn back on after quitting the game was irritating me.

Disclaimer: It's been a couple of years since I've written any Rust code, so this probably isn't the most elegant implementation.

There don't seem to be any automated tests, so I tested manually by starting and quitting a game with the music playing, which correctly paused and then unpaused the music, then pausing the music and starting and quitting a game, which correctly left the music paused.

[jerith]

Having two slightly different copies of the music state update logic was bothering me, so I factored it out into a method on MusicCommand. The same manual test I did earlier still passes.

[MCOfficer]

ended up using #360 instead, sorry for the radio silence