rework pause mode

- game is now paused if the board loses focus (e.g. if a menu is opened or the application itself loses focus)
- game can now be resumed by clicking the board

There is a strange timer issue that needs to be resolved.
If you pause the game and then choose to play a new game, the timer has the negative pause time. o.O
But that's a different matter and the timers are on my list anyway. Too many type casts... :)