Week 4: Doors and Keys

Added a box that spawns keys above it every five seconds, and added a Door, Yellow Key, and Yellow Door.

Normal keys can be collected and added to your total. As long as your total is above 0, you can open the regular door, and it takes 1 from your total. The Yellow Key can be collected once, and once you have it, you have it forever. This key is used to open the Yellow Door.