CategoryMembershipArityConstraint should have always been a string
  to allow defining a TALES Expression as already used in filesystem
  Property Sheets.

* Add use_acquisition attribute to allow merging CategoryAcquiredExistenceConstraint
  into CategoryExistenceConstraint, and CategoryAcquiredMembershipArityConstraint into
  CategoryMembershipArityConstraint.



git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@42928 20353a03-c40f-0410-a6d1-a30d3c3de9de