Game Over + High score added

A proper game over state is added
- Pauses the game
- Displays the high score
- Ask player if he wants to restart or quit

High score added
- High score properly added
- High score saved and loaded from a file
- There shouldn't be any errors if the file is missing or the high score
isn't inside.
- There's no validation if the highscore was saved as a non-integer
though