Analogous to QWidget::setAttribute(), introduce an API to easily
enable/disable a single window flag without having to resort to
w.setFlags(w.flags() | Qt::NewFlag).
Change-Id: Ib0f7254a34c8d884cdec181c41b99e5ef035d954
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Sérgio Martins <sergio.martins@kdab.com>