Follows the QWindow semantics, and is a replacement for creating
a QWidget with a QDesktopScreenWidget as the parent.
We can now remove much of the special handling of QDesktopWidget and
the Qt::Desktop window type, and get rid of QDesktopScreenWidget.
Add a manual test that allows local testing. Our CI environments
only have a single screen, and no multi-head display server setup
which is the primary case where QWidget::setScreen is interesting.
For the more common case of a virtual desktop, QWidget::setScreen
has no real impact (just as QWindow::setScreen doesn't).
Change-Id: Id0099e069d316741bacd8c795c396ccad37be297
Fixes: QTBUG-85483
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
c54a5b8380