Board editor castling rights checkboxes not updated with FEN

[AAArmstark]

A user created study chapters from 960 starting positions and expected the castling rights to be included, because they were selected initially, but they were gone.

I've tried to reproduce the problem and noticed the following:

1. Open the board editor (it also happens in the study chapter board editor with or without 960 variant selected)
2. Change the starting position for both players so that castling is not allowed normally (for this example, swap the rook on a1/a8 with the knight on b1/b8)

In the FEN, the 0-0-0 castling rights are removed, but the corresponding castling rights checkboxes are still ticked.

Additionally, now unchecking the already from the FEN removed 0-0-0 castling rights for White, adds the 0-0-0 castling rights for Black back to the FEN without the user actually changing anything to Black's castling rights settings.

0-0-0 rights missing from FEN but checkboxes still ticked

Edit: #12929 may be related but I don't know.

[kraktus]

#13453 might fix it I think