NBT data is now stored per world per dimension

In Minecraft/Forge, each dimension is not its own World (unlike Bukkit).
An example better explains:

Minecraft:
    - New_World
        NORMAL
	THE_END
	NETHER

Bukkit:
    - New_World
    - Nether
    - The_End

Seeing as this is a Forge mod and we do the base game, Backpack data is
now stored in:

ExtendedInventory -> New_World -> NORMAL/NETHER/THE_END -> Slots

TODO Sharing of backpacks between dimensions and worlds.

Signed-off-by: Chris Sanders <zidane@outlook.com>