Add QtQml.Models module

The moved Model classes can now be exposed in a QtQml import.

To keep the QtQml import resticted to more core functionality, they are
being exposed in a plugin module.

Change-Id: I0a84642a72c7c9bbf9b6ffd2a6c33549f8e61c29
Reviewed-by: Alan Alpert <aalpert@blackberry.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>