Fix calling .remove() on views.
Fix calling .remove() on views.

This depends on https://github.com/jupyter/notebook/pull/2411 to get all the corner cases. We left in various other event triggers to cover most cases even without that PR.
SylvainCorlay/ipywidgets,jupyter-widgets/ipywidgets,jupyter-widgets/ipywidgets,ipython/ipywidgets,jupyter-widgets/ipywidgets,SylvainCorlay/ipywidgets,ipython/ipywidgets,ipython/ipywidgets,SylvainCorlay/ipywidgets,ipython/ipywidgets,ipython/ipywidgets,jupyter-widgets/ipywidgets,SylvainCorlay/ipywidgets