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/mod.rs

@@ -2,6 +2,7 @@
 
 #[cfg(target_os = "macos")]
 mod appkit;
+mod notification_center;
 #[cfg(not(target_os = "macos"))]
 mod uikit;