Update existing message, support groups, rename test suite
The (session_id, timestamp, text)
triplet
is the key even though it isn't in SQLite.
Updating messages by that should become
important when we start polling messages
from the server, especially if we don't
ACK, and if we implement attachments later.
Groups are also supported to some extent.
Tests were split so that the qt-model tests get to have the model-related name.