Finished the basic single player system

- Single player now works, it might need some polish later on but for
now it is functional.
- Some functionality was added to the SinglePlayerScreen class to set
when the after game text and the new game button are visible.
- I have also implemented an AI system which is clever enough to stop
the user from winning if it needs to and also win when it can.
- The AI can’t think more than one turn into the future though.