Initial revision of task_workflow. Permissions added. Simplified variable handling (which were not that useful, the previous idea was that the user would enter at each transition the confirmed date, which is nice but not consistent with the rest of ERP5. Also, state_change['object'] was not accessible at object creation)

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@14840 20353a03-c40f-0410-a6d1-a30d3c3de9de