Add Expand/Collapse tooltips to expand/collapse arrows for better accessibility

[JenPeleva]

Describe any alternatives/workarounds you're currently using

We need to query each node and add the attribute ourselves, which isn't very convenient.

Other information:

If accepted, I would be willing to submit a PR for this feature

[ x ] Yes (Assistance will be provided if you need help to submit a pull request)
[ ] No