Backpack: Ton of Stuff:
* Changed auto-centering to leave room for empty slots when inventory is open
* Consolidated and restructured slot logic, added swapping and shifting, added event-based dragging
* Fixed character resetting issues
* Made dragging only enabled while open, and closing only allowed when nothing is being dragged
* Fixed slot number showing below frame when dragging
* Added background frame while dragging
* Added drag-and-drop feature for swapping between hotbar slots, from an inventory slot to a specific hotbar slot (empty or not), and from hotbar slots to the inventory
* Made drag-and-dropping outside of the backpack area just move the tool to the inventory to copy current backpack behavior - later this will drop the tool
* Fixed leaving behind hidden inventory slots after a swap
* Added inventory frame rearrangement when a gap is created from dropping/swapping
* Made some visual changes to match mockup
* Cleanup and other minor improvements