Bump accesskit to 0.18 and make it a workspace dependency (#5783)

This can't be merged until [kittest's accesskit is
bumped](https://github.com/rerun-io/kittest/pull/9) ~~and [a new version
of rfd is released](https://github.com/PolyMeilex/rfd/pull/240)~~.

---------

Co-authored-by: Lucas Meurer <lucasmeurer96@gmail.com>

crates/egui-winit/src/lib.rs

@@ -166,12 +166,14 @@ impl State {
     #[cfg(feature = "accesskit")]
     pub fn init_accesskit<T: From<accesskit_winit::Event> + Send>(
         &mut self,
+        event_loop: &ActiveEventLoop,
         window: &Window,
         event_loop_proxy: winit::event_loop::EventLoopProxy<T>,
     ) {
         profiling::function_scope!();
 
         self.accesskit = Some(accesskit_winit::Adapter::with_event_loop_proxy(
+            event_loop,
             window,
             event_loop_proxy,
         ));