Code clean up:
- New private methode "KBBGame::comfirmGameEndIfNeeded()" to avoid duplication of code.
- Several redundant tests "if (running)" removed.
- Small clean up in "KBBGame::giveUp()".
- Division of the class (and file) KBBGame in 2 classes (and 2 files) to remove a lot of dependencies to the graphic widget (KBBGraphic) in the main window:
- KBBGame: main window of the game
- KBBBoard: logical board of the game

- Avoid to annoy the player with the confirmation of the end of the game when he didn't really start to play.