Adds a simple Backpack to Minecraft

Features:
 - 36 slots
 - 2 Tool slots
 - shows the Armor slot
 - shows the Offhand slot

Usage:
 - press 'B' to open the Backpack
 - press 'N' to get the Tool from the upper Tool slot
 - press 'V' to get the Tool from the lower Tool slot
 - press 'G' to cycle through the Tool slots (cycle: upper -> lower -> hotbar)
 - press 'SHIFT' and scroll with the mouse wheel through the Tool slots

API support with file XBackpack-1.18.1-1.1-API