[backport#15040] Add workaround for QProgressDialog bug on macOS
Summary:
See: QTBUG-65750, QTBUG-70357.
Backport of Core PR15040
This apparently reapplies a nit fixed in D2144. I've chosen to stay close to the Core version for less merge headaches.
Test Plan: unfortunately I do not have a macOS in hand to test this
Reviewers: #bitcoin_abc, jasonbcox
Reviewed By: #bitcoin_abc, jasonbcox
Differential Revision: https://reviews.bitcoinabc.org/D6957