Fix bugs in handling of refunds and additional payments for event registration

As described here (and recreated by me):
http://forum.civicrm.org/index.php/topic,33752.msg143150.html#msg143150

Create a new participant of an event..
Refund their event contribution..
Change selections..
Record a payment..

The payment records and balance are valid, but the existing contribution record still shows 'Refunded' status. The contribution status should be updated to 'Completed' reflect the result of the last action - in this case it should be completed. See comments below for additional use cases.