QmlDesigner.Model: Allowing the model to hold a TextModifer

We have a one to one relationship between models and TextModifiers.
With this patch the model keeps track of its own TextModifer and
does set it to the rewriter if attached.

The model does not take ownership, since it does also not take
ownership of its views.

Change-Id: Ied02666debf6aad76666b0d9d22e8ba2f72be666
Reviewed-by: Marco Bubke <marco.bubke@digia.com>