Merge: iOS support

Merge: iOS support

Adds a target platform in the compiler which generates an XCode project for iOS applications.

Intro the `ios` projet to group services for iOS. It implements the callbacks expected by the iOS system for a basic graphical application. It also triggers compilation for the iOS platform.

Adds one example of a graphical application `hello_ios` and a very minimal test for the iOS platform.

This is still very much a work in progress, you will notice many TODO in the code. Here are some of the next features to implement:
* Extend support to all iOS devices, not only the iPhone in the simulator.
* Reorganize annotations for app.nit: app_name, package_name, and maybe organization_name.
* Add more services of the Cocoa touch library and rewrite the `hello_ios` app in Nit.
* Support assets and other game related features.
* Some cleanup of the generated XCode project files could be a good thing.

Pull-Request: #1227
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Romain Chanoir <chanoir.romain@courrier.uqam.ca>