GUI: Refactor actual QR code rendering into new QRImageWidget::setQR

Summary:
Pull request description:

For at least QR-code based pairing of mobile wallets with nodes, it will be desirable to render QR codes even without wallet support.

Therefore, this prepares by moving the QRImageWidget out of a wallet-specific file into its own qrencoder file-pair.

This is part 1 of 2 of backport of Core PR15928
Commit 77851ab6

Test Plan:
ninja && ninja check && sudo ninja install
Run bitcoin-qt and make sure the QR code is still generated properly.

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7783