Make sure QWidgetPrivate::hide_sys() really hides

Make sure QWidgetPrivate::hide_sys() really hides

When commit 55fa3c189f88933d390177ad5606d3de9deacf93 was merged from
api_changes, the conflict resolution left it possible for hide_sys() to
leave a visible window on screen. This happens when
Qt::WA_DontShowOnScreen is set on a visible widget. hide_sys() needs to
always hide the platform window if it is non-zero.

Change-Id: I3a1a882b66954e90d6ba80c657de69cae32e75a7
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>