End of Game Detection

The GameModel should provide a way to detect end of game conditions. Checkmate and king capture are the two ways to remove a player from a game. Once both players on a team (1 and 3 for team A, 2 and 4 for team B) are removed, the other team wins. As a further note, unlike normal chess, this variant of 4-player chess allows players held in checkmate to be returned to play if the threat to the king is removed.