bug fix to ensure only logic is needed

This update is a small bug fix where some players reported that levels where sometimes generated with very few mines. This mostly happened in expert mode.

This was due to the internal solver that attempts to complete the level with logic only (so that you can do, no luck needed). And if the level was not possible to complete, it would remove some mines. In some cases, it would remove way too many mines.

So this should now be sorted, or at least very much improved. But if you ever see levels that cannot be solved, I encourage you to post screenshots in the discussions.