[backport#15886] Show recipient list as detailedText of QMessageBox

Summary:
https://github.com/bitcoin/bitcoin/pull/15886/commits/2ee756f0411c467fc638373ea68d6cb5ced46974

Depends on 7289

Partial backport of Core PR15886

Test Plan:

ninja

send coins with bitcoin-qt -regtest and check it behaves as expected

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7290