Inventory update.

Changed the inventory background to an image a friend of mine (allyally)
made. Also changed items to have a render function instead of having
many things implement rendering them to allow for easier management of
icon animations. Changed inventory controls, swapped 2 and 3 around to
prevent accidental deletion when moving items. Added skeleton entity
(unfinished and doesn't spawn yet) to prepare for testing item use code.
Items that wear out now show a wear bar under them in the inventory and
show a percentage remaining at the top. Weapons now show their damage
range in the form of dice rolls and added constants (i.e. 3d4+5).
Current version will not run due to lack of images for items.