Fix focus handling and add a changed signal
Fix focus handling and add a changed signal

jupyter/jupyter-js-plugins,jupyter/jupyter-js-plugins,blink1073/jupyter-js-plugins,blink1073/jupyter-js-plugins,blink1073/jupyter-js-plugins,blink1073/jupyter-js-plugins,blink1073/jupyter-js-plugins,jupyter/jupyter-js-plugins,jupyter/jupyter-js-plugins,jupyter/jupyter-js-plugins