[Classic notebook] Add event listener to window.resize for top-level Phosphor widgets

[SylvainCorlay]

Jupyter Widget views are phosphor widgets.

When displayed in a classic notebook output area, they are not notified for window resize events.

We should probably update the widgetsnbextension to add these event listeners.

[SylvainCorlay]

cc @jasongrout @vidartf

[jasongrout]

Let's make sure this is happening.

[jasongrout]

@ibdafna is working on this at #3124

[jasongrout]

Fixed in #3124.