Group AccessKit nodes by Ui (#7386)

* closes https://github.com/emilk/egui/issues/5674

This changes egui to create an AccessKit node for each `Ui`. I'm not
sure if this alone will directly improve accessibility, but it should
make it easier to create the correct parent / child relations (e.g.
grouping menus as children of menu buttons).
Instead of having a global stack of parent ids, they are now passed via
a parent_id field in `UiBuilder`.

If having all these `GenericContainer` nodes somehow is bad for
accessibility, the PR could also be changed to only create nodes if
there is actually some accessibility info with it (the relevant is
currently commented-out in the PR). But I think screen readers should
just ignore these nodes, so it should be fine? We could also use this as
motivation to git red of some unnecessary wrapped `Ui`s, e.g.
CentralPanel creates 3 Uis when 2 should be enough (the initial Ui and a
Frame, maybe we could even only show the `Frame` if we can give it an
UiBuilder and somehow show the Frame with `Ui::new`).

Here is a screenshot from the accessibility inspector
(https://github.com/emilk/egui/pull/7368) with this PR:

<img width="431" height="744" alt="Screenshot 2025-07-24 at 12 09 55"
src="https://github.com/user-attachments/assets/6c4e5ff6-5c38-450e-9500-0776c9018d8c"
/>

Without this PR:


https://github.com/user-attachments/assets/270e32fc-9c7a-4dad-8c90-7638c487a602

crates/egui_kittest/tests/accesskit.rs

@@ -18,9 +18,20 @@ fn empty_ui_should_return_tree_with_only_root_window() {
 
     assert_eq!(
         output.nodes.len(),
-        1,
-        "Empty ui should produce only the root window."
+        4,
+        "Expected the root node and two Uis and a Frame for the panel"
     );
+
+    assert_eq!(
+        output
+            .nodes
+            .iter()
+            .filter(|(_, n)| n.role() == Role::GenericContainer)
+            .count(),
+        3,
+        "Expected two Uis and one Frame as GenericContainer nodes.",
+    );
+
     let (id, root) = &output.nodes[0];
 
     assert_eq!(*id, output.tree.unwrap().root);
@@ -35,12 +46,6 @@ fn button_node() {
         CentralPanel::default().show(ctx, |ui| ui.button(button_text));
     });
 
-    assert_eq!(
-        output.nodes.len(),
-        2,
-        "Expected only the root node and the button."
-    );
-
     let (_, button) = output
         .nodes
         .iter()
@@ -61,12 +66,6 @@ fn disabled_button_node() {
         });
     });
 
-    assert_eq!(
-        output.nodes.len(),
-        2,
-        "Expected only the root node and the button."
-    );
-
     let (_, button) = output
         .nodes
         .iter()
@@ -86,12 +85,6 @@ fn toggle_button_node() {
         CentralPanel::default().show(ctx, |ui| ui.toggle_value(&mut selected, button_text));
     });
 
-    assert_eq!(
-        output.nodes.len(),
-        2,
-        "Expected only the root node and the button."
-    );
-
     let (_, toggle) = output
         .nodes
         .iter()
@@ -114,12 +107,6 @@ fn multiple_disabled_widgets() {
         });
     });
 
-    assert_eq!(
-        output.nodes.len(),
-        4,
-        "Expected the root node and all the child widgets."
-    );
-
     assert_eq!(
         output
             .nodes
@@ -194,15 +181,25 @@ fn assert_window_exists(tree: &TreeUpdate, title: &str, parent: NodeId) -> NodeI
 }
 
 #[track_caller]
-fn assert_parent_child(tree: &TreeUpdate, parent: NodeId, child: NodeId) {
+fn assert_parent_child(tree: &TreeUpdate, parent_id: NodeId, child: NodeId) {
+    assert!(
+        has_child_recursively(tree, parent_id, child),
+        "Node is not a child of the given parent."
+    );
+}
+
+fn has_child_recursively(tree: &TreeUpdate, parent: NodeId, child: NodeId) -> bool {
     let (_, parent) = tree
         .nodes
         .iter()
         .find(|(id, _)| id == &parent)
         .expect("Parent does not exist.");
 
-    assert!(
-        parent.children().contains(&child),
-        "Node is not a child of the given parent."
-    );
+    for &c in parent.children() {
+        if c == child || has_child_recursively(tree, c, child) {
+            return true;
+        }
+    }
+
+    false
 }