PayPro QR code receive payment standards implementation

Follow up to: #587

Add full implementation of PayPro draft standards for Selene Receive QR payments (and possibly sending as well). May need to be reworked over time with the standard still not finalised, but we can help push it forward by implementation, adding urgency to needed changes and review. See: https://github.com/bitjson/chip-paypro

Perhaps to be done by Jim as part of CashStamps improvements.