Bugfixes, proper Audio settings and Highscore

Hi, just got back from vacation   ┐(￣∀￣)┌, noticed some bugs and fixed them up, added some small QOL stuff. Going back to AI behaviour and optimisation after this in the coming days

Bugfixes:

• Total time now properly resets when starting a time trial (forgot to reset it after finishing the time trial)
• Total time now calculated properly - previously it wouldnt add the last lap time to the total
• Audio - the audio now scales linearly from 0% to 150%, default is 100%. Percentage shown in settings

Adjustments:

• Edited the walls on the Glacier map a bit so they are more forgiving, you can go under some walls for a bit as if they are hanging/cliffs
• Physics process now handled on a seperate thread, should have improved performance by a tiny bit

Additions:

• Record total and lap times are now saved between sessions! 
• Each map now displays its FASTEST COMPLETION TIME and FASTEST LAP TIME records
• Settings are now saved between sessions!
• Added a setting to turn off the OVERHEAT WARNING sound effect