[macOS] QMessageBox doesn't look native

QMessageBox does not look like a native NSAlert dialog on Mac OS X.

• QMessageBox displays close/minimise/zoom widgets in the title bar, but a modal dialog should not have these widgets (not even in a disabled state).
• QMessageBox packs the window contents too tightly compared to the standard NSAlert layout convention.

• QMessageBox makes the buttons too small compared to the standard NSAlert button widths.

• QMessageBox does not display the application icon
• and much more

Related (but closed because Qt4 is no longer supported): QTBUG-4949 QTBUG-9481

Possible idea of implementation (mine): https://github.com/kleuter/qcocoamessagebox

Comparison screenshots are attached.

QMessageBox::information(nullptr, "Error", "This is the error message");

vs

QCocoaMessageBox::information(nullptr, "Error", "This is the error message");