[progress tracker] tree view backed by nestable selectable evented lists

[alisterburt]

I'm opening this issue to track progress towards achieving the following

I can imagine us getting to a place where we could say "you tell us what your tree structure looks like and we'll give you a UI for it".

I think we want the model for this to be a nested set of SelectableEventedLists in psygnal - in superqt we can provide a view of this model similar to the QtTreeView in napari

• [x] tlambert03/psygnal#64 to psygnal
• [x] tlambert03/psygnal#78
• [ ] add support for nesting to the evented list in psygnal
• [ ] make Qt view classes

You can probably tell I'm a little vague on the details in the last step 😆