Include mixins when reading document type parent definitions

org.onehippo.cms.channelmanager.content.documenttype.ContentTypeContext#populateTypesForFields

This method reads the field definition for an editor template node, from its own document definition or from parent definitions.

We should add here that we also read mixin definitions if available. Example: the tags functionality. In the document type editor add the Taggable from the Mixins tab on the right.

The mixinloader plugin class is used, and the "mixin" property defines the mixin used. Its definition at /hippo:namespaces/hippostd/taggable can be looked up and used. I think this mixin can then actually be supported, since the field definition is a multiple string field!

But if after lookup of the mixin the conclusion is that we cannot yet support it (e.g. hippotaxonomy:classifiable), that must be reported as an unsupported field in the tracking messages.