Automatically disable buttons when animations are playing

[jkomoros]

After fixing #396, the state you currently see rendered might be old, while you wait for animations to play out. But the buttons to propose moves are still there, and sometimes when you propose a move when you think the state is a certain way, it's actually another way.

Ideally it would be something that each game renderer could decide to listen to or ignore, just a isAnimating property (although ideally all of the buttons would automatically get the right styling and disabled treatment, and you'd opt out of it).

Another option is to just ignore propose-move when it comes while animating is true.