Do not announce when getting carbon messages
Being notified on carbon messages may be a disturbance.
Having Gajim running on more than one computer I kind of disturbing ([GajimChat:2015:03:28:13:15:42 arune]). I am using a mobile client, sitting in the kitchen and hear the ping ping ping all the time from next room ([GajimChat:2015:03:28:13:16:37 NUF]).
Add a switch to prevent gajim from making announcements when getting carbon messages.