Allow engine to force user to make non-standard promotion

Allow engine to force user to make non-standard promotion

The engine can now force an unexpected promotion on moves entered
by the user, by marking the to-square of such a move in blue.
Releasing a piece on such a blue square will then defer further
processing of the move (after having send the 'put' command to the
engine) until reception of a 'choice' command to specify what the
piece should promote to. This solves problems in Maka Dai Dai Shogi,
where a piece capturing a contageous piece would only be allowed to
promote to its normal promoted form if the to-square was marked in
purple (offering a P / +P toggle).