Add BattleController with functioning battle flow

There's lots of sample code, like modals if you try to do something when it's not your turn, and a duck-button to cast spell. Spells don't really work because they always assume they should apply to hero1, so both players think only they get affected by the spell. Also, Bird52 doesn't kill units (and it doesn't take effect until after 1 round, but I think that's fine). Passing and forfeiting might or might not work.

Lots of stuff is moved from BattleState, and some should be moved into BattleView.

Closes #29 (closed)