Mojave dark mode tasks

Copied from QTBUG-70346:

qtquickontrols2/tests/manual/testbench

• Fusion style button text is dark with low contrast

https://codereview.qt-project.org/#/c/251609/

• Controls do not pick up “live” light/dark theme changes, app restart is needed.

QTBUG-73435

I'm not sure if this one is possible currently, and I wouldn't consider it a high priority.

For my own reference, I think the change happens here: http://code.qt.io/cgit/qt/qtbase.git/tree/src/plugins/platforms/cocoa/qcocoatheme.mm?id=8915904e2a56d46bdedf64f9a7a5e331ae9d00e1#n159

QGuiApplicationPrivate::notifyThemeChanged() also emits qGuiApp->paletteChanged(*app_pal), which might be relevant.

• None of the styles pick up the system accent color.

QTBUG-73453

https://codereview.qt-project.org/#/c/221775/ is an example of adding a new palette role, though care must be taken not to introduce the same bug as QTBUG-72555.

• Otherwise OK: fusion style dark mode and the dark versions of Material/Universal work well