Add empty fields

[phylor]

Preliminary version to add blank fields to the QR bill.

I already post this here for others to give feedback. I'd like to first test it in reality though.

Todos

• Currency/amount in the payment part is positioned a bit low. Maybe position it at the same height as the one in the receipt section. However, this is dynamic, e.g. if we don't have a reference.
• I believe there's too little space for the "acceptance point". Maybe we have to get rid of the creditor street in the receipt section (check spec if it defines the address there).
• Layout uses flex, which might be an issue if people still use wkhtmltopdf (as the QT backend doesn't know flex yet), maybe change to use tables instead?
• Add a test for the QR payload