Slow [Alpha 3]

This update adds the slow element. It's a walker that moves every 2nd turn.

There's also quite a few bug fixes and improvements:

• Added animations for all elements that may do something "on the next turn". Consistent with the new slow element, virus, spawn, and other animations.
• Updated swap sound effect. A lot of things swap so the noise needed to be more subtle.
• Key bugs: Editing & undo could result in incorrect key count (this is stored separate from the keys visible to save on sweeping the whole map each turn). Powerups can clear keys which wasn't being counted.
• Path finding: Path finding flood respects walls and blockers like boxes, the active flood doesn't - so active creatures could end up without AI info. Path finding flood has been doubled to compensate.
• Rollers were waiting for path info to move which they don't need.