Defer book faking input move until ping balance

Defer book faking input move until ping balance

The use of ping after editing a position had broken the book handling:
'go' would be sent in the same batch of commands as this 'ping', so that
in case of a book hit the book move selected on behalf of the engine
would be fed back to XBoard before the engine had the opportunity to
restore the ping balanceby sending 'pong'. Now the book move is fed
through ScheduleDelayedEvent(DeferredBookMove) rather than instantly,
when a MachineWhite/BlackEvent sets the engine playing.