Notebook: restore cell editor view state

[rebornix]

Notebook is rendered in a virtualized list in the core and when you are scrolling the document, cells are inserted and removed based on the scrolling position and viewport. If you edited a code cell and scroll to other places in the document and then scroll back, you would expect the code cell is in the same sate as before.

Currently it's not:

[rebornix]

Fixed via ad22aa2