Slate Shops

First release (suggested by Architect and Iammaliikk).

• One currency for the server, in whole numbers. Its name, plural and symbol and the balance a new player starts with (100) are set in config/slate/shops.json. /money (or /balance) shows yours, /pay <player> <amount> sends money, and operators give, take and set balances with /money give|take|set.
• An item can back the money (exchangeItem and exchangeValue, off unless you set them): /money deposit turns those items into money and /money withdraw turns money back into items.
• Server shops, made and edited by operators on the shops screen's Server shops page, and on the same page of Slate Server's administrators' menu when it is installed. An offer is an item with all its data, how many a trade gives, a price and an optional price the shop pays to buy it back. Their stock never runs out.
• Player shops: /shops create <name> (or the My shop page) opens your own, with up to 27 offers made from the items you carry. Its stock is real items: /shops stock shows it, /shops stock add and /shops stock take fill and empty it. Buyers pay the owner, also while they are away; a shop that buys pays from its owner's balance. /shops close and /shops open close it for a while.
• Screens in Slate's look, in both styles: Browse (every shop, with a search and a filter for server and player shops), My shop, Wallet and, for operators, Server shops. /shops and the Slate hub open them. A player without the mod trades through chat: /shops lists the shops with names to click, /shops view <shop> shows the offers with [Buy] and [Sell].
• Nothing is lost or doubled: a trade runs whole on the server, one buyer after the other, and checks the room in the buyer's inventory first. An offer or a shop with stock in it cannot be removed until the stock is taken out. Shops and balances are saved with the world, as plain JSON in <world>/slate/shops/.
• Permissions slate_shops.use, slate_shops.create and slate_shops.pay (everyone), slate_shops.admin and slate_shops.money (operators); with Slate Server, its roles give and take them. A command another mod already has (/money, /pay, /balance) is left to it, and Slate's is then typed /shops money or /shops pay.

Slate modules need the Slate Core of their own version: update all of them together.