Empty-to-draft transition can be executed by assignor (which previously executed draft-to-stop transition).
Draft-to-stop transition can be executed by assignee.
Deleted can only be reached from empty state.
Draft state can now reach cancelled state.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@19536 20353a03-c40f-0410-a6d1-a30d3c3de9de