Fix desync when viewing history on engine's turn

If you use the history buttons while the engine was thinking, we would
get an "oops! something has gone wrong" once the engine decided to move.
Return view to the present when engine moves

If we are looking back in history, force the view to return to the
present as soon as the engine moves.
ChessGame: do not complete move if game is over

Fixes clock unpausing when using Undo after gnuchess has decided to
report a move but before it has actually done so (which is no longer
possible after the previous commit, anyway)
Prepare 3.12.3