Add toggle-expand-all command to toggle folding of nested elements

[dakra]

Analog to the existing toggle-expand command but toggle-expand-all also shows/hides all child elements.

PR Note: I couldn't find a function to extract all child locations from the current context, maybe there's a better/easier way than the get-child-locations function.

[djblue]

Hi @dakra, thanks for the PR! I think for expanding children, the incremental approach in https://github.com/djblue/portal/pull/189 is what I prefer. Still need to test out and work through some bugs, but let me know if that approach works for you.

[dakra]

Thanks. I'll close this PR and experiment a bit if that's enough for me or if I should make a new PR for toggle-all with the new logic.

A few observations of your PR vs mine:

• obviously I still have to hit shift+e a few times to fully expand a nested map
• If I expand a certain path and collapse one of it's parent, the folding state of it's children are now lost. i.e. I have to expand the elements again.
• If I expand (or expand all and collapse again) some children and the call expand-inc on a parent, it does nothing. I liked the way this PR does it i.e. if not ALL children are expanded, it does an expand, otherwise (== all children showing) it collapses.

Thanks again :)