Feature: Refactoring Drag-and-Drop interface elements

**UI Package**
* `DroppableElement` changed to `Slot`
* `DraggableElement` changed to `Draggable`
* UIHandler 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`