make: use static libraries and sfml-main
boxField: calculate which boxes to press correctly

Previously algorithm pressed all non-mine boxes touching the
one that user pressed. This behaviour was wrong, since clearly boxes
touching the pressed box diagonally shouldn't be touched.
boxField: calculate correctly touched mines

In order to avoid incorrect indexing, previously boxes in the border had
wrong number on them.
boxField: add coloring for digits
boxField: refactor private functions
boxField: setup mines after the first press
boxField: try to divide mines more evenly
boxField: allow winning
boxField: show flagged mines correctly after game over
README.md: remind about arial.ttf
fix winning condition checking