Now only redraws changed parts of the level. This gives a speed up (try hitting 'C')
and allowed fixing a redraw bug for 'games/mahjongg -f'.

The second larger change is an undo feature ('U' or 'Bksp').

Other changes:
- added a mostly useless command 'deselect' so carelessly
	right-clicking doesn't end the game
- some purely logical code moved from graphics.c to the new logic.c
- tried to make some parts of the code clearer