* Add CategoryProperty Document, especially relevant when the accessor
  generation will be moved from Utils.
* Add Reference to property_sheets in Documents as 'reference' will be
  used instead of title or id.
* Move all Documents to ERP5Type.Core as there should have always been there.



git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@39408 20353a03-c40f-0410-a6d1-a30d3c3de9de