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/event_handler.rs

@@ -124,7 +124,7 @@ impl EventHandler {
                 callback(*user_app, event_loop);
             },
             Ok(None) => {
-                // `NSApplication`, our app delegate and this handler are all
+                // `NSApplication`, our app state and this handler are all
                 // global state and so it's not impossible that we could get
                 // an event after the application has exited the `EventLoop`.
                 tracing::error!("tried to run event handler, but no handler was set");