Added show valid moves button to play mode. All functionality is now
ready as it was before, just it is tidy and clean now! It is not
exporting to a file yet, @Lukas Vascila should do it soon. Just need to
add a method to GoPlaying Board. Also there are no labels which were
before - I will leave it to @Lukas Vascila if he still wants them. So,
now it is possible to switch between two different modes: Play and Build
problems. I feel very great!