Restore Widget state when opening a notebook connected to a live kernel

[DonJayamanne]

• Open a notebook, run a cell with widget output against a local kernel
  • Split the notebook editor (the new editor will not have the widget displayed)
• Open a notebook, run a cell with widget output against a remote kernel
  • Close the notebook and re-open it.
  • Even though the remote (active) kernel is auto selected, the widget is not displayed

We need to restore the Widget output from the kernel
This is possible for IPyWidgets 8, and possibly for later versions of IPyWidgets 7 (need to test and see if the kernel protocols are supported)

[DonJayamanne]

Duplicate of #16288