* Implement field generation on the first click - now you won't blow up from start :-)
I don't think that it should generate the field in a way so the first click is on an empty cell - this would be too easy IMO.
Currently it just ensures, that first click is not on mine.
Objections?

* Drop old "digits-assigning" algorithm, use new - much faster one which doesn't iterate over whole field

* Some code cleanups