Refactoring Inventory Package, using UI package

UI Package
* `DroppableElement` changed to `Slot`
* `DraggableElement` changed to `Draggable`
* UI Handler interfaces have been created for Slots, and Draggables
* `UI` now supports a more Unity-like API

Inventory Package
* `InventorySlotHandler` and `InventoryDraggableHandler` have been
created to handle events received from `UI.Draggable` and `UI.Slot`
Updating Package

Meta's for `Spawner` were also re-created.