Idea: Add payment feature

[saerdnaer]

It would be nice if the fnordload could also accept money e.g. for payment of members fee's or other things like t-shirts.

• New menu entry 'payment' etc
• dialog asks how much money you want to spend
• note acceptor takes money
• dispenser gives out the rest in coins
• receipt code is printed and shown at display
• receipt code can be shown to finance officer
• finance officer can mark receipt codes as used