[Legacy] Trade Booths

This is a bugfixed version of the Trade Booths mod, originally by aeroc.
I don't even know what I fixed anymore, but I'm told it was important. Therefore, here it is, after four years of slumber.

This is a legacy project. No updates.

The Trade Booth Mod adds a pair of interactive blocks that facilitate 24-hour offline/away-player trading, especially useful for servers.

A Trade Booth is a two-part structure that must be built together. First you need to create and place a storage block. Once placed in the world, storage blocks are claimed by the first person to use them. Once claimed, they cannot be opened or destroyed by other players and are used to store items bought and sold in transactions. Like the storage block, the top block is claimed by the first person to use it after it is placed in the world. The top block must be on top of a storage block owned by the same player in order to be traded.

The owner can move stacks of items to any of the top block's 16 slots, which represent 8 different transaction options. In each pair, the item in the left slot is the price for the item in the right slot.

Customers can transact by simply left-clicking on the right slot of any slot pair. The block's logic will verify that the transaction is possible by checking that the matching items are present in both the warehouse and the player's inventory, and that there is space for the items to be traded.