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

crates/egui_demo_app/src/wrap_app.rs

@@ -183,6 +183,10 @@ impl WrapApp {
         // This gives us image support:
         egui_extras::install_image_loaders(&cc.egui_ctx);
 
+        #[cfg(feature = "accessibility_inspector")]
+        cc.egui_ctx
+            .add_plugin(crate::accessibility_inspector::AccessibilityInspectorPlugin::default());
+
         #[allow(unused_mut, clippy::allow_attributes)]
         let mut slf = Self {
             state: State::default(),