Ability to create new variants

[Feliks-WR]

Wouldn't it be nice to able to create our own variants?
So we can play stuff like reformed courier-spiel, sovereign chess, Capablanca chess, et cetera?
Or 4 player xiangqi?

[QueensGambit]

There was a draft PR about s.th. similar: #229
This could be interesting after some more urgent issues have been fixed.