For animation, moves should contains "moving" and "fading" maybe...
(But it's really just for Magnetic chess)
setInterval "CRON" task in sockets.js to check connected clients
(every 1hour maybe, or more)
Systematically show init+dest squares in PGN, maybe after short notation
(2 moves list, second for de-ambiguification)
Button to show all pieces that can move (next to expert mode; change icons)

Bugs, limitations:
 - switching chess when castling: show 2 promotion kings (??)
 - Crazyhouse: TODO = keep track of promoted pawns.
 - Checkered: implement stage 2 ?!