Widgets going "undercover" should also undercover the QWindow

Sometimes Qt wants to hide a widget without calling setVisible(false).
This is usually done by setting the widgets geometry outside the
parents clip rect (and usually by setting its y coordinate to a
sufficiently negative y coordinate). QSplitter uses this when it needs
to collapse its children.
Previously the QWindow was just moved straight above the QWidget it was
hosted in.

Task-number: QTBUG-38475
Change-Id: I154dd4d13f108c3d34c64eadb41dd6b477dc5c4e
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>