Cell folding that respects the user foldexpr

[GCBallesteros]

@cnrrobertson This is very much WIP. Code is a mess and is missing all kind of sanity checks but the core idea is there

[patnr]

Jumping in here as well! Would this support automatically nested folds?

In long notebooks (paired with Jupytext) it is common include several levels of section headings (usually # ## ### etc). It would be great if this could be detected. Accordingly, the foldtext would display the heading.

I guess it might be complicated to support different Jupytext formats and section heading markers. Perhaps the user would have to chose the light format and set the cell_markers Jupytext option .