Called when a property has changed.

def notify_change(self, change):
        """Called when a property has changed."""
        # Send the state to the frontend before the user-registered callbacks
        # are called.
        name = change['name']
        if self.comm is not None and self.comm.kernel is not None:
            # Make sure this isn't information that the front-end just sent us.
            if name in self.keys and self._should_send_property(name, getattr(self, name)):
                # Send new state to front-end
                self.send_state(key=name)
        super(Widget, self).notify_change(change)