Track index in Pref Tree to facilitate automatic scrolling

[colin-grant-work]

What it does

Fixes #9532 by tracking the index in the tree rather than using the index from the model. Previously, the index used for automatic scrolling didn't match any node in the TreeWidget's display.

Some additional cleanup of the filtering process is possible, but I prefer to defer that to a separate PR after #9439 is merged.

How to test

1. Expand nodes in the Preference UI tree so that the next expansion could push the node up out of view. (see GIF in The menu jumps when it is expanded in preferences. #9532).
2. Expand a node.
3. Observe that it is not pushed out of view.

Review checklist

• as an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• as a reviewer, I agree to behave in accordance with the review guidelines

[colin-grant-work]

@shuyaqian, would you mind testing this to ensure it fixes your problem?

[vince-fugnitto]

I confirmed that the tree expansion works much better following the changes.
When expanding nodes in the tree, the automatic scroll no longer pushes the expanded node outside the view 👍