Set stockfish depth

[SamuelWiet]

Cool tool to play blindfold. But stockfish can't beat me with default depth 3 even with maximum skill level. So I add the depth to setting. Sorry, without automatic test - I have not such react skills and did not found any test belonging to the settings.

[cgoldammer]

Let me look at this later this week. I like the idea, but it's a bit non-trivial to test so I'll have to think if I can find a way of landing this safely.