AI Improvement, Cheat Codes and Difficulty Selection

Changelog:

• Improved the AI: it now takes some effort on my side to beat it, which means it does more than it should. For this reason...
• Added difficulty selection: as I said earlier, you are not supposed to play the CPU except at first to learn the game. Since the newest CPU was definitely over what I was aiming for, I decided to restore the old AI and have it as the basic CPU, with the new one being the Expert CPU.
• Several cheat codes added: they are definitely my new hobby now.
• Minor bug fixes: minor because they were only accessible with cheat codes, but now they are overall fixed. I doubt we'll encounter anything gamebreaking in the near future.