Create 6 Shopkeeper payment integration

[adam8797]

This PR implements a workflow for users to both make and accept payments using numismatics coins (or cards) at Create shopkeepers.

Player Experience

Enabling the shopkeeper to accept coins

Opting into these features is as simple as placing a coin depositor next to the same stock ticker that serves the shop. It can be placed on any orthogonal side, like so:

The player may then optionally place a card in the depositor. If they do not insert a card, coins will be accepted but they will remain in the depositor.

Placing an order as a customer

If a customer attempts a transaction that does not include any numismatics coins in its costs, the player will not experience any difference. The standard Create shop mechanics will take over and fulfill the order.

If the order does have costs in the form of coins, they'll be prompted with a payment screen:

They can then place a card in the slot, or pay with coins out of their inventory. The Pay with Coins button is disabled if they do not have the requisite amount of coins available in their inventory.

Upon clicking one of the payment buttons, the transaction is processed, and then any non-coin item costs are processed in the standard way.

If the player pays with coins, we withdraw coins in a greedy fashion, and will break a coin into change if needed. This is a one-click coin payment, regardless of the number and variety of coins needed.

This patch does allow for payment of mixed item & coin, but each individual item can still only have a cost of a single item or coin.

Technical details

The main flow here is that of a deferred payment system. We intercept calls to the Create interactWithShop() function which is called whenever the player right clicks on a stock keeper with a shopping list. We check a handful of preconditions, and if they all pass we:

1. Save the context of the current transaction, assign it a unique ID, and register it in a global registry, and cancel the underlying Create function call.
2. Pop the menu open for the player to interact with, and they make a selection of payment method
3. Once the selection has been made, we send a packet back to the server to complete a given deferred transaction.
4. If the user abandons the menu, we send a packet back to void the deferred transaction.

resolves #119