Fix crash when connecting a new screen

In QWaylandWindow::virtualSiblings, don't include screens that have not been
added yet. I.e. QWaylandScreens for which QPlatformIntegration::screenAdded has
not yet been called.

The cherry-picked version does not include the tests from the original patch.

Task-number: QTBUG-62044
Change-Id: I623fbf8799d21c6b9293e7120ded301277639cc6
Reviewed-by: David Edmundson <davidedmundson@kde.org>
Reviewed-by: Aleix Pol
Reviewed-by: Paul Olav Tvete <paul.tvete@qt.io>
(cherry picked from commit fd9fec4f)