Finally Stockfish AI is implemented

It's done, the next big update was just uploaded. After problems with refactoring my code and several weeks of illness, now you can play against stockfish, with is strong enough to challenge everyone reading this ;)

Newly added features:

• Play against stockfish AI
• Choose between different colors
• The pieces able to move are marked to make it more beginner friendly
• 1 new map implemented

Removed bugs:

• You can't eat the opponent king anymore
• You can't put your king in check
• Game ends with Checkmate
• Castleing works properly

Minor changes:

• Changed the Map/Color-Choose UI
• Changed the GameOver-UI
• The AI gets different names, depending on the difficulty
• You loose the game if your time runs out