Disconnect from socket listener signals when leaving main window

https://code.briarproject.org/briar/briar-gtk/blob/c345f861eac8dcd871d3a880ea0c6e4098d1478e/briar-gtk/briar_gtk/containers/main_window.py#L153

    def _load_content(self):
        self._contacts = Contacts(APP().api)
        self._load_contacts()
        # TODO: Disconnect if no more needed
        APP().api.socket_listener.connect("ContactAddedEvent",
                                          self._refresh_contacts_async)
        APP().api.socket_listener.connect("ConversationMessageReceivedEvent",
                                          self._refresh_contacts_async)