Automatically add the parent Ui as accesskit parent

[lucasmerlin]

It'd be great if children of a Ui could automatically have that Ui as a parent in accesskit. If that's too much of an overhead, it'd be nice if this could be an opt in via UiBuilder.

Describe alternatives you've considered
Using the current Ctx::with_accessibility_parent is cumbersome and unintuitive. (See here)

Additional context
This would make it easy to implement correct accessibility for containers (e.g. radio groups) and also makes it easy to query children of containers in egui_kittest.