Positional inventory support

[bsergent]

Support positional inventories as one implementation of Inventory where items can take up multiple slots. Store an array of item locations and sizes, and then populate a map of all slots and whether they're occupied or not for finding new placements. Look at new projects page of my website for examples. This would also require the ability to rotate items within the inventory. The difficult part will be getting this to work with UIInventory's arbitrary slot positions.

See #8