Store additional mixins added through the DocumentType Editor in a separate (new) mixins property instead of the supertypes property

Currently additional mixins like taxonomy:classifiable are also saved as a 'supertype' in the supertypes property of a hipposysedit:nodetype.
However the behavior and semantics of such mixins is different from 'real' supertypes which are actually stored and defined in the explicit JCR nodetype definition as supertypes.
Such additional mixins are only applied/added to prototypes, as mixin, and therefore not part of the type system for the document primary type.
The ContentTypesService (CMS7-7024) must separate these semantically and model-wise, so it would be better and easier to also store such additional mixins separately (e.g. separate property hipposysedit:mixins) in the hipposysedit:nodetype.