chess 6.x-1.0-beta2

by Tim_O: Fixed missing workflow for ending and ended games.

Games could not be surrendered. To address this:
* the checkmate-checkbox-handling was modified, so it can be ticked
without actually making any move.
* the checkmate-checkbox is now also used to surrender a game
Ended games were not treated as such, i.e. still allowed to enter a
move. To address this:
* when displaying node an according info is shown, if the game has
ended (and no move or undo form, i.e. ending a game is irreversible)
* rules-integration was extended by a "game-ended"-action including
an according default-action

Should be fully functional and ready for release now?