Polishing the setup details

Now, the setup has its own method and it's capable to compute player's option to player against AI or not.

Furthermore, there are two ways to restart the game:
- when a game finishes;
- when the `Restart Game` button is pressed.

In the first case, we don't need to prompt the user asking if he wants to play against AI again – it's annoying and I think it was decreasing UX. By the bye, when the button is pressed then we ask about the user if he wants to play against AI or multiplayer.

I think there are some improvements to be done, but basically everything is working fine.