[Inspector V2] RichText widgets show up in inspector tree

[elliette]

Fixes #8103

I noticed that the widget tree didn't contain any RichText widgets. This is because there was a flaw in the logic determining whether an implementation widget should be in a hidden group. We weren't handling the following two cases:

1. The implementation widget is "sandwiched" between two visible widgets, e.g.

Project Widget A
            |
Implementation Widget B
            |
Project Widget C

1. The implementation widget is the child of a visible widget, and has no children itself (like RichText), e.g.:

Project Widget A
            |
Implementation Widget B

[elliette]

Not yet ready for review, converting to draft PR.