added QR code for simple payment by bank transfer

The one payment method that doesn’t incurs any fees for us to pay is SEPA bank transfer.
Commonly used in Germany and Austria.

Up to now customers had to copy&paste multiple fields (IBAN, Beneficiary, sum, order number) to their online banking to pay.
Since all online-banking apps now support this, we added a QR code that can be scanned from the screen of your desktop computer using your phone instead.