This is how the fake screen currently behaves in the xcb plugin. So if we are
to deduplicate, it's probably best to match current behavior first.
I still left an option for not being a virtual sibling in case other platforms
don't support showing a window on a placeholder screen.
Task-number: QTBUG-79711
Change-Id: I4e8b44d892efb85fdb003f1d473d0867442d7e4e
Reviewed-by: Paul Olav Tvete <paul.tvete@qt.io>
f0a43a9def