QmlDesigner: create DesignerSupport on stack

 - it has the same life cycle like the Qt5NodeInstanceServer
 - we don't need to create and delete it (with maybe memory leaks)
 - this fixes a crash while the puppet is closed after an item deletion

Change-Id: Iecd6515eb41324b95e99e151d77ee74895230182
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>