GUI: Move QRImageWidget to its own file-pair

Summary:
Commit fc929842
This concludes backport of Core PR15928
Depends on D7783

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, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7794