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).
