[MERGE] OPW 18001: do not lock-out user when payment.order is 'done', allow it to...

  - When 'Payment Order' is in 'done' state, the workflow is frozen and the
    user is unable to cancel it (required in case bank refuse the payment).
    This allow to cancel the payment order except when some lines are already
    imported in a bank statement (i.e payment done bank-side: modification
    forbidden).

bzr revid: xal@openerp.com-20120306140039-5r8l3xrzbaq70xev