Add the mini-grids to Board::validate
[1/2] Make the Board::Validate invalid tests more compreheensive

I still need to find some better invalid full boards for the tests
UI fix =)
Simplify minigrid validation
Make Solver::solve void, not bool
Rewrite solver's logic to allow for more flexibility
Add valgrind's log file to gitignore
Optmise Solver::clearPossibilities (and add some unit tests for it)