Using a callback to display a sent message inside the ChatControl means
that all messages we send have to be issued from the GUI layer
(send_message()) if we want them to display in the ChatControl.

This replaces the callback and catches the stanza-message-outgoing event
after it was processed by the core.

This is easier to read/understand than dealing with callbacks and lets
the core issue messages without having to care if a ChatControl is open or not