Implement a few things

- Backpack now respect language file entities for Keybindings.
- Backpack opens a container, size 9 for now
- Backpack saves to/loads from NBT

This means the core structure of Backpack works fine. Need to add features
missing from the plugin such as upgrades, blacklisting, and world sharing.

Permissions will be added when Sponge is in use with its permissions API.
Packet support is in place to support this when Sponge is ready.

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