servoshell: Enable accesskit integration. (#37519)

These changes ensure that our browser shell can integrate with
screenreaders. We do not provide any accessibility information about
webview content yet, which requires further API design work in both
Servo, accesskit, and egui.

Testing: No a11y-specific testing at this point; just verifying that
existing tests continue to pass.
Fixes: part of #4344

---------

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

ports/servoshell/desktop/headed_window.rs

@@ -92,7 +92,9 @@ impl Window {
             .with_transparent(no_native_titlebar)
             .with_inner_size(LogicalSize::new(window_size.width, window_size.height))
             .with_min_inner_size(LogicalSize::new(1, 1))
-            .with_visible(true);
+            // Must be invisible at startup; accesskit_winit setup needs to
+            // happen before the window is shown for the first time.
+            .with_visible(false);
 
         #[allow(deprecated)]
         let winit_window = event_loop