Time Trial!

In this update, I added:

• Time Trial! - A new gamemode where you have to complete as many laps as you can in 1 minute.
• Achievements - Achievements now pop up in the bottom right corner every time you achieve something, for example - 'Getting under 18s on Grass Track' or 'Play Multiplayer'
• Car improvements
• Better highscore management - now all highscores are stored in 1 json file, instead of seperate txt files.
• General code refactoring and bug fixes.