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