Fix rendering glitches when using native widgets in MDI subwindows

Fix rendering glitches when using native widgets in MDI subwindows

When mixing native and regular widgets in same QMdiArea, some
subwindows didn't properly get set native. This was because
when a native parentless widget was given a parent, it wouldn't
enforce native window on the new parent and its ancestors.
This happened because window flags were adjusted too late in
relation to createWinId() call in setParent_sys().
Fixed by moving the createWinId() call to its proper place.

Also removed some old Q_WS_* ifdeffing in QWidget::setParent() that
masked some native enforcement code.

Additionally removed few QEXPECT_FAILs from QWidget autotest now
that those cases work correctly.

Task-number: QTBUG-26424
Change-Id: Ib6f9d0531e5c7299e2c307734d49c81f1ffa9713
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>