Message carbon: do not notify carboned messages
Bug description
When two clients, Gajim and another client, are online, the non Gajim client sends out a message would notify the gajim message like a normal incoming message.
Steps to reproduce
Log in the same account with gajim and an arbitrary client C. The server and client C must have XEP 0280 enabled. Use C to send out a message. The gajim client would get this carboned message but it notifies like an incoming message from non-self.
Software versions
OS version: Arch Linux
GTK version: 3.16.2
PyGTK version: 2.24.0