Factor out incoming message processing

Split out common code that is used for adding an incoming chat message
to a conversion.

Signed-off-by: Lukas Fleischer <info@cryptocrack.de>
Fix author retrieval for group chat images/sounds

Get the actual author instead of using uninitialized data for the author
name if an image, sound or location is sent to a group.

Signed-off-by: Lukas Fleischer <info@cryptocrack.de>
Merge pull request #46 from cryptocrack/group-receive-image-fixes

Group receive image fixes