Destroy the QWindow children in destroy() instead of close()

Destroy the QWindow children in destroy() instead of close()

When closing a window, destroy() ensures a setVisible(false) call only
for the window itself, not for other windows parented to it. With the
new quit lock ref feature this breaks code that creates a fake root
window parented to the main, visible window. (for example the
xcomposite backends of qtwayland do this)

Such apps do not anymore exit after closing their window because the
children do not receive a deref due to setVisible not getting called.
(At that point. It would get called after exiting the event loop but
that never happens due to the refcounting)

Change-Id: I124737c80ad59600ddc79261100f3904af0f410d
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>