Document class should have Reference property sheet, that is the core of document management.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@43654 20353a03-c40f-0410-a6d1-a30d3c3de9de

product/ERP5/Document/Document.py

@@ -290,6 +290,7 @@ class Document(DocumentExtensibleTraversableMixin, XMLObject, UrlMixin,
                     , PropertySheet.CategoryCore
                     , PropertySheet.DublinCore
                     , PropertySheet.Version
+                    , PropertySheet.Reference
                     , PropertySheet.Document
                     , PropertySheet.ExternalDocument
                     , PropertySheet.Url