Add return value to with_accessibility_parent (emilk#5083)

Extracted out of emilk#4805

In [`egui-theme-switch`] I'm allocating a response inside the closure
passed to `with_accessibility_parent` so that my radio buttons have the
radio group as parent.

I'm working around the lack of return value with a custom extension
trait for now: [`ContextExt`]

* [x] I have followed the instructions in the PR template

[`egui-theme-switch`]:
https://github.com/bash/egui-theme-switch/blob/main/src/lib.rs
[`ContextExt`]:
https://github.com/bash/egui-theme-switch/blob/main/src/context_ext.rs

Author: bash

crates/egui/src/context.rs

@@ -2887,9 +2887,9 @@ impl Context {
     /// the function is still called, but with no other effect.
     ///
     /// No locks are held while the given closure is called.
-    #[allow(clippy::unused_self)]
+    #[allow(clippy::unused_self, clippy::let_and_return)]
     #[inline]
-    pub fn with_accessibility_parent(&self, _id: Id, f: impl FnOnce()) {
+    pub fn with_accessibility_parent<R>(&self, _id: Id, f: impl FnOnce() -> R) -> R {
         // TODO(emilk): this isn't thread-safe - another thread can call this function between the push/pop calls
         #[cfg(feature = "accesskit")]
         self.frame_state_mut(|fs| {
@@ -2898,14 +2898,16 @@ impl Context {
             }
         });
 
-        f();
+        let result = f();
 
         #[cfg(feature = "accesskit")]
         self.frame_state_mut(|fs| {
             if let Some(state) = fs.accesskit_state.as_mut() {
                 assert_eq!(state.parent_stack.pop(), Some(_id));
             }
         });
+
+        result
     }
 
     /// If AccessKit support is active for the current frame, get or create