with preference is correctly initialized before calling
  _aq_dynamic on a portal type using class Preference. This solve
  issues when _aq_dynamic was called on a Preference before
  any need of _aq_dynamic on PreferenceTool
* Move Priority class from ERP5Form.Document.Preference to
  ERP5Form.PrefrenceTool in order to solve circular imports

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@36176 20353a03-c40f-0410-a6d1-a30d3c3de9de