Allow finish game in example mines

[xilec]

Allow finish game in example mines.

Added:

• detection of victory.
• showing mines, at the end of game.
• generation of board after first player click, for not losing on first step.

Not implemented: reveal nearby unmarket cells by middle mouse click and count scores.
There is known bug: do not shows mines when game is defeated. But almost the same implementation for victory case works.

[gyscos]

Closed in favor of the larger PR doing the same thing