Disconnect from socket listener signal when closing private chat

https://code.briarproject.org/briar/briar-gtk/blob/c345f861eac8dcd871d3a880ea0c6e4098d1478e/briar-gtk/briar_gtk/containers/private_chat.py#L82

    def _load_content(self):
        [...]
        # TODO: Disconnect if no more needed
        APP().api.socket_listener.connect("ConversationMessageReceivedEvent",
                                          self._add_message_async)