Visual Editor: Unsupported field types check omits mixin fields

The VE notifies the user if a document type has fields that VE cannot yet show. The check is only done on fields defined in the section hipposysedit:nodetype/hipposysedit:nodetype/* of a document type definition.

But fields that use the MixinLoaderPlugin to define a field, are only configured in the editor:templates part. These type of fields must be included in the check for unsupported fields as well.

Example: the taxonomy selection field can be present in a document type, but is not identified as a unsupported field type by the VE.