(Un)Register a custom msg receive callback.

        Parameters
        ----------
        callback: callable
            callback will be passed three arguments when a message arrives::

                callback(widget, content, buffers)

        remove: bool
            True if the callback should be unregistered.

def on_msg(self, callback, remove=False):
        """(Un)Register a custom msg receive callback.

        Parameters
        ----------
        callback: callable
            callback will be passed three arguments when a message arrives::

                callback(widget, content, buffers)

        remove: bool
            True if the callback should be unregistered."""
        self._msg_callbacks.register_callback(callback, remove=remove)