Allow the user to register the application delegate on macOS and iOS (#3758)

This allows the user to override the application delegate themselves,
which opens several doors for customization that were previously closed.

To do this, we use notifications instead of top-level application delegate
methods.

One effect of not providing an application delegate on iOS is that we no
longer act as-if the application successfully open all URLs there.

This is a breaking change, although unlikely to matter in practice, since the
return value of `application:didFinishLaunchingWithOptions:` is seldom used by
the system (and is likely the preferred behaviour anyhow).

src/platform_impl/apple/appkit/window.rs

@@ -28,9 +28,8 @@ impl Window {
         attributes: WindowAttributes,
     ) -> Result<Self, RootOsError> {
         let mtm = window_target.mtm;
-        let delegate = autoreleasepool(|_| {
-            WindowDelegate::new(window_target.app_delegate(), attributes, mtm)
-        })?;
+        let delegate =
+            autoreleasepool(|_| WindowDelegate::new(&window_target.app_state, attributes, mtm))?;
         Ok(Window {
             window: MainThreadBound::new(delegate.window().retain(), mtm),
             delegate: MainThreadBound::new(delegate, mtm),