update all data of existing messages in extend()

update all data of existing messages in extend()

when the message identifier (msgid or context/source/disambiguation)
appears in multiple places, the remaining meta data may be inconsistent
between the occurrences. we need to handle partial data on one hand, and
contradicting data on the other.

Task-number: QTBUG-29131
Task-number: QTBUG-32725
Change-Id: I1ed08e42e266b0b5f014400c137abbcec9ffb93d
Reviewed-by: hjk <hjk121@nokiamail.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>