qt: Replace objc_msgSend with native syntax

Summary:
Fix the build on MacOS Catalina (require D4734).

Backport of core PR16720:
https://github.com/bitcoin/bitcoin/pull/16720/files

Test Plan:
Build on OSX.
Run the OSX gitian build.

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4735