TODO

   1 For animation, moves should contains "moving" and "fading" maybe...
   2 (But it's really just for Magnetic chess)
   3 setInterval "CRON" task in sockets.js to check connected clients
   4 (every 1hour maybe, or more)
   5 Systematically show init+dest squares in PGN, maybe after short notation
   6 (2 moves list, second for de-ambiguification)
   7 Button to show all pieces that can move (next to expert mode; change icons)
   8
   9 Bugs, limitations:
  10  - switching chess when castling: show 2 promotion kings (??)
  11  - Crazyhouse: TODO = keep track of promoted pawns.
  12  - Checkered: implement stage 2 ?!