* cls._categories contains all categories associated to an object.
  This is list is aggregated and contains all categories associated
  by a property sheet, and all categories coming from the portal
  type definition itself.
* Categories defined on property sheets get their accessors generated
  and assigned on accessor holders.
* But categories defined on the portal type need to be generated
  distincly, on the portal type class itself. When doing so, we must
  generate accessors only for portal type categories, and ignore
  those already generated.


git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@43362 20353a03-c40f-0410-a6d1-a30d3c3de9de