QmlDesigner.Instances: always set the QObject parent.

We follow the QmlEngine/VME and always set the QObject
parent before reparenting into list properties.

Also we set the QObject parent for all objects.

See qqmlvme.cpp line 622
QQml_setParent_noEvent(o, parent);

Change-Id: If889e6cc07e99ea443b80de13c4be9dce6c2ceee
Reviewed-by: Marco Bubke <marco.bubke@digia.com>