Gravity Grid compability feature

[5reddman]

Coolest thing in the world: https://chess.berkefiliz.com/

To make many of the levels auto-checkable*, it would be enough to have two features:
Condition "gravity" - after each move, everything that isn't nailed shut falls down until
no falls are possible anymore (luckily, the order is irrelevant. Only after that, further
computations set in.
Piece property "hover" - Piece is nailed shut

• In "Gravity Grid" all black pieces have property capture-only; kings are captured