Began moving drag and drop elements to a `UI` package

* These were currently inside the `Inventory` package
* UI Event Handlers will need to be created to handle custom events for
elements in the `UserInterface` package- this will allow the decoupling
of the `Inventory` and `UserInterface` package
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.
Merge pull request #1 from adampassey/dev

Feature: Refactoring Drag-and-Drop interface elements