[backport#16944][gui] watch-only wallet: copy PSBT to clipboard

Summary:
https://github.com/bitcoin/bitcoin/pull/16944/commits/c6dd565c8820aa8a98b190621e10c6e2821a9ecc

Depends on D7295

Concludes backport of Core PR16944

Test Plan:

ninja all check check-functional

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7727