DocumentTypeEditor doesn't retain mixins on a base type when creating a new type extending that base type

As an example the hippotaxonomy:classifiable mixin defined on a base type (as a supertype but actually only 'applied' as mixin on the prototype and instances of documents) is not copied as super type when you create a new type extending the base type. This results in a 'broken' model where the super type has more mixins as the sub type.