Report egui::Window contents as children to accesskit

[Wcubed]

Previously, all widgets would be listed in accesskit as children of the toplevel window.
With this change, they will be reported as child of the egui::Window they are in, Which should increase parseability of the ui for screenreaders and integration tests.

Added an accesskit test to check that it is indeed working.

[github-actions]

Preview available at https://egui-pr-preview.github.io/pr/5240-accesskit_window_parent
Note that it might take a couple seconds for the update to show up after the preview_build workflow has completed.

[lucasmerlin]

Awesome!
I also stumbled upon this in my work on #5166, I think it'd actually make sense if all children of a ui are automatically added as a child of that ui in accesskit? So e.g. if you create a ui.group() with a button, that button should be child of the group?

Also, once #5166 is done we should be able to simplify the accesskit tests using kittest

[Wcubed]

That was indeed my idea as well 😄.