Fix accidental revealing of items when the game is restarted after lost from a middle click.

Summary:
When a flag is misplaced, a middle click could cause the game to lost.
Currently, if the user choose to restart the game in this case, there is a high chance
that a few pieces around the one middle clicked are revealed right after the game restarted.

The reason is that when the user restart the game in such case,
it happens within checkLost/onItemRevealed which means that the
game might not be the same one anymore after onItemRevealed returns
and the loop handling all neighboring pieces are now operating on the now restarted game.

The fix is to make sure the loop stops when the game is finished. Checking m_gameOver is
unreliable and won't work in this case since the restart of the game would have reset it.
Instead, the finished status is returned from all functions that trigger such a condition
and the information is propagated back to the caller to terminate the loop appropriately.
This also improved another unreliable use of m_gameOver that is currently harmless.

Debugging was possible thanks to rr.

Reviewers: mlaurent, yurchor, KDE Games, aacid

Reviewed By: yurchor, aacid

Subscribers: aacid, kde-games-devel

Differential Revision: https://phabricator.kde.org/D27716