Support to download the Output Descriptors in QR format

[maxirosson]

The idea is to add support for downloading 3 QRs (BCR-2020-010 encoding) with the Output Descriptors of a multisig. Those 3 QRs should be compatible with the SeedHammer QRs stamped on the metal plates, that you are already supporting to import. So, with this feature users will be able to store/distribute the Output Descriptors of a multisig with more privacy, because someone should find 2 of those QRs in order to see the funds of your wallet.