QmlDesigner: Fix rubberband

It should reparented because at a model detach it will be removed from the
scene and deleted.

Task-number: QTCREATORBUG-12504
Change-Id: Id48c8d8f8a29b7aab60dca708bbefdac8c1f6024
Reviewed-by: Alessandro Portale <alessandro.portale@digia.com>