OSX: do not force a plain window to be on the same level as its parent

2af0a778f464980594c36098e4a8ba4448edfd29 the fix for QTBUG-27410
caused this Designer bug.  Doing the automatic level escalation only
for "special" windows and avoiding it for plain Qt::Window type
windows is one way of fixing the Designer problem.

Task-number: QTBUG-31779
Change-Id: I1da5454f31111f36480fac3b53be6d5f0ce40047
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>