Refactor state flags

[lewster32]

A lot of the game's state is contained in flags, which is less than ideal, as many (most?) combinations of flags are invalid. This screams 'finite state machine' as a solution (I've heard mention before that games and UI should be practically all driven by FSMs) so let's look at that shall we? There are already several enums (including an unused one on the Piece class) which can be used for this purpose.