Need to create new object in order to sync changes

[ianhi]

See example of jupyter-widgets/ipywidgets#2916

in widget_message_speed I extended an array with concat to make sure it would sync: https://github.com/ianhi/widget_message_speed/blob/cdd7f5b9a23d1c219e566feea7240ee2dc008150/src/widget.ts#L43