messagelistmodel: improve moveMessages

+ moveMessage was doing useless computation to move messages one
by one even if blocks are supported
+ beginMoveRows/endMoveRows seems to do weird stuff if all the rows
should be moved causing some crashes. beginReset in this case
doesn't trigger this weird behaviour and is more efficient.

Change-Id: Ia4eb3cdbbe74bf9215fb673cb9af571f67225ffe