point_of_sale: workflow erroneous validated in add payment

Affects version 5.0 and trunk. Line 437 of point_of_sale/pos.py the pos workflow is erroneous validated:

        wf_service.trg_validate(uid, 'pos.order', order_id, 'payment', cr)

The signal must be 'paid' instead of 'payment', like line 225 of the same file:

        wf_service.trg_validate(uid, 'pos.order', order.id, 'paid', cr)

Now the pos payments are working luckily because, in the payment wizard, after calling add_payment() (that validates erroneously the workflow) is always called test_order_lines() (that validates the workflow correctly).