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/uikit/mod.rs

@@ -1,6 +1,5 @@
 #![allow(clippy::let_unit_value)]
 
-mod app_delegate;
 mod app_state;
 mod event_loop;
 mod monitor;