New chill difficulty (version 19)

Added a new difficulty: chill. The timer does not tick down, and pieces do not rain down unless you hit the button. It starts on the easy floors, then medium then hard.

I decided that having an actually easy mode is good. There are still explosions, so it's actually still pretty scary (basically, I think I still haven't gone far enough).

If someone wanted, they could still time attack it and/or go for all clears (in fact it's pretty hard to all clear, so I think this mode could be fun for fast players too, in a different way).