Moved some files around. Added level boundaries. Added main menu.

Sorted some GUI scripts to their respective folders. Added a boundary
around every level so that the user cannot walk off the edges. Also
added a (temporary) title screen for the game.
Fixed vertical jumping speeds

When walking up a hill, the vertical speed is positive. Thus, jumping
would add onto the current speeds, and would then send the user
skyrocketing into the air. Fixed this.