Renamed _ghostFunc to _ghostify, removed LockAction prompt, added _lockAction config.
Remove unused constructor.
Get rid of annoying (a) parameter in Piece.
Clarified some code.
Allow prompt to be disabled.
(Or more accurately, allow it to be a constant prompt.)
Disable prompt when using simulation to compute the ghost piece.
(The code currently doesn't hit a prompt path as of now, but let's future-proof it.)