IPyWidgets not rendered when switching renderers

[DonJayamanne]

1. Create a cell with the following code

import ipywidgets as widgets
widgets.IntSlider()

2. Run the cell
3. Confirm slider is displayed
4. Switch renderer to plain text
5. Switch back to widget renderer & nothing is displayed.

We should be able to re-render the widget as we have all of the information.
We need to account for the fact that the widget was disposed & it needs to be re-rendered.

[DonJayamanne]

Changing to debt as no one has reported this as fixing this is a good architectural cleanup (i.e. debt)

[DonJayamanne]

This can be fixed with IPyWidgets 8,
However closing this issue as its unlikely anyone would change renderers
We haven't had users reoprting this (except internally for those testing renderers)