Reworking interface and API for UI Handlers

* `SlotHandler now sends an `ItemDropped` event instead of a `MouseUp`
event. Returning `true` from the handler will consume the currently
dragged item
* `DraggableHandler` now sends: `Hover`, `Drag`, `Click`, `ItemDropped`.
Returning a `DraggableItem` will begin dragging it
* Interaction with `DraggedItem` Singleton is now done via `Draggable`
and `Slot` instead of the individual handlers
`InventoryGUI` is now constructed via Factory
Worn equipment will now display if `Equipment GUI` is hidden
UI Handlers no longer pass `Event` as argument
`EquipmentItem` now supports rendering layers

* Rendering layers are based on the `EquipmentType`. The further up the
type appears in the declaration, the further back it will be during
render.
* Updating `Equipment` so `EnableItem` and `DisableItem` handle all item
modifications for it to be equipped or unequipped.
`EquipmentGUI` is now constructed via Factory
Updating Unity Package, saving scene
Fixing `NullReferenceException` when non equipment was dropped in equipment slots.
Initial pass at `ActionBar` package

* `UI.Slot` now takes an optional `GIUContent`
Adding `ActionBarController`

This controller will activate items in the `ActionBar` associated with
their number.
Merge remote-tracking branch 'origin/master' into gh-pages
Updating scene, re-creating `Red Potion` prefab
Updating Unity Package with `ActionBar` components
Merge remote-tracking branch 'origin/master' into gh-pages
Updating Example to include `ActionBar`