Accessibility inspector plugin (#7368)

Adds an accessibility inspector plugin that shows the current AccessKit
tree:


https://github.com/user-attachments/assets/78f4f221-1bd2-4ce4-adf5-fc3b00f5c16c

Macos has a built in accessibility inspector, but it doesn't seem to
work with AccessKit / eframe so this provides some insight into the
accesskit state.

This also showed a couple issues that are easy to fix:
- [ ] Links show up as `Label` instead of links
- [ ] Not all supported actions are advertised (e.g. scrolling)
- [ ] The resize handles in windows shouldn't be focusable
- [ ] Checkbox has no value
- [ ] Menus should have the button as parent widget (not 100% sure on
this one)


Currently the plugin lives in the demo app, but I think it should be
moved somewhere else. Maybe egui_extras?

This could also be relevant for #4650

Cargo.lock

@@ -1331,6 +1331,8 @@ dependencies = [
 name = "egui_demo_app"
 version = "0.32.3"
 dependencies = [
+ "accesskit",
+ "accesskit_consumer",
  "bytemuck",
  "chrono",
  "eframe",