Auto save/backup when Output changes

[DonJayamanne]

When user executes cells in a notebook, the output changes, at this point the Notebook should be changed.

At the end of the day output is part of the Notebook Document stored on disc.

Similarly, if the execution order changes or similar metadata changes, the document should be saved.

Challenges
I guess we'll need to determine what metadata contributes to saving a notebook document, or more specifically what metadata can be stored in a document. Technically all metadata can be stored in a physical file, hence if any metadata changes, then document must be saved to disc.

However, making a cell runnable/editable doesn't necessarily have to be saved to disc, at least not in the case of Jupyter notebooks.

Alternate solution - extension can be responsible for performing the save/backup as and when it sees fit.

@rebornix /cc