SeedQR for Bitcoin Core hot wallet

This is especially useful when experimenting with the [SeedQR proposed standard](https://github.com/SeedSigner/seedsigner/blob/dev/docs/seed_qr/README.md).

It would be nice if the desktop app can render the seed as a (compact) QR code, and vice versa: scan a (compact) QR code to get the mnemonic words. The latter feature should come with a stern warning _not to do that_ with any mnemonic that belongs to a real hardware wallet.

Start by reading the linked SeedQR proposed standard and inspect how the desktop app currently handles wallet mnemonic input and display. The feature is done when the app can render a mnemonic as a compact QR code and scan one back into mnemonic words, with a stern warning against using real hardware-wallet seeds.

Written by the indexing model from the issue text.