Better AI, knockback, and code enhancements

Another semi-major change to the code:

- added knockback effects
- improved item code
- added a leftClick and rightClick method for items
- started work on a particle effects system
- improved accuracy of projectiles
- made platform borders visually thicker
- fixed entities falling through platforms
- if you are outside the range of an enemy it will not follow you unless
you attack it
- changed inventory slot image so the transparent center is 32x32
- projectiles do not rotate past the weapons range (ex. bow can only
shoot 45 degrees in the air, so arrow sprite will not rotate paste 45
degrees)