Post-jam update: music and some level fixes

I've updated the game with a post-jam version with a few changes:

• Thanks to Patricia Taxxon for making background music.
• Some levels have been edited to remove unintentional solutions. Mostly these were minor changes, but level C2 has been completely reworked.
• Different floor heights have different ground and border textures.
• Elevators are now blue instead of red (previously crates were similarly coloured).