Improve discoverability of cell folding

[kieferrm]

We provide actions for folding cells. This is hard to discover. Jupyter supports double clicking on the focus bar which is rather intuitive. We don't have a focus bar, but we should explore whether we can find another way to support double click to fold/unfold cells.

[miguelsolorio]

I wonder if we could get away with double-clicking the empty space on the side. We do this double-click in other areas (explorer, suggest widget, find widget):

collapse-cells.mov

cc @rebornix @roblourens

[roblourens]

Implemented this, only works for code cells currently and only on the left side of the sidebar, needs #119535

[miguelsolorio]

@roblourens I'm not able to get this to work, do you have a screen recording?

[roblourens]

I forgot to merge the notebook/dev branch with this change. It's not in the build yet...