Merge: Intro support for async HTTP requests and ListLayout for iOS

This PR completes the implementation app::UI for iOS by implementing `ListLayout` using a `UITableView`. It also introduces the implementation of `http_request` using services from the foundation framework.

These features enable the Tnitter portable application to be compiled for iOS without modification.

The remaining API of app.nit still missing on IOS are `data_store`, `audio` and maybe `assets`.  At this point, only `data_store` is required for fully featured portable applications using the native UI.

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As with #1942, some generated code has been commented so it can be reactivated when useful and when the needed types are also wrapped.

Pull-Request: #1943
Reviewed-by: Jean Privat <jean@pryen.org>