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/src/id.rs

@@ -83,6 +83,25 @@ impl Id {
     pub(crate) fn accesskit_id(&self) -> accesskit::NodeId {
         self.value().into()
     }
+
+    /// Create a new [`Id`] from a high-entropy value. No hashing is done.
+    ///
+    /// This can be useful if you have an [`Id`] that was converted to some other type
+    /// (e.g. accesskit::NodeId) and you want to convert it back to an [`Id`].
+    ///
+    /// # Safety
+    /// You need to ensure that the value is high-entropy since it might be used in
+    /// a [`IdSet`] or [`IdMap`], which rely on the assumption that [`Id`]s have good entropy.
+    ///
+    /// The method is not unsafe in terms of memory safety.
+    ///
+    /// # Panics
+    /// If the value is zero, this will panic.
+    #[doc(hidden)]
+    #[expect(unsafe_code)]
+    pub unsafe fn from_high_entropy_bits(value: u64) -> Self {
+        Self(NonZeroU64::new(value).expect("Id must be non-zero."))
+    }
 }
 
 impl std::fmt::Debug for Id {