(Un)Register a widget displayed callback.

        Parameters
        ----------
        callback: method handler
            Must have a signature of::

                callback(widget, **kwargs)

            kwargs from display are passed through without modification.
        remove: bool
            True if the callback should be unregistered.

def on_displayed(self, callback, remove=False):
        """(Un)Register a widget displayed callback.

        Parameters
        ----------
        callback: method handler
            Must have a signature of::

                callback(widget, **kwargs)

            kwargs from display are passed through without modification.
        remove: bool
            True if the callback should be unregistered."""
        self._display_callbacks.register_callback(callback, remove=remove)