[Bleeding] Return correct furnace SlotType. Fixes BUKKIT-5400

Previously, the SlotType for the 0 slot in a furnace returned CONTAINER,
when it should have returned SlotType.CRAFTING. This updates the code for
determining slot type to return the proper value.
[Bleeding] Return correct player SlotType. Fixes BUKKIT-3188

Previously, the SlotType for the last 4 slots in a player's inventory
returned QUICKBAR when it should have returned SlotType.CONTAINER. This
updates the code for determining slot type to return the proper value.