improved internal solver to avoid moves requiring luck

There has been several feedback recently of situations where there would be no move available. For instance here, here and here.

Ideally, the generated levels should be solvable without having to rely on luck, so there clearly was something I had not implemented. I've then taken another look at the internal solver, that simulates how a player would uncover or flag tiles. And it wasn't dealing with walls of mines very well. Instead of removing mines in that wall, it would get stuck and revert to removing mines randomly.

This updates fixes that, so now hopefully we should be at a stage where no luck is required to solve any level.