Merge: iOS icon support and fix black bars on physical devices

Merge: iOS icon support and fix black bars on physical devices

This PR adds support for custom icons on iOS by adding an `Assets.xcassets` folder to the generated Xcode project. It is used to store a user provided icon specified in the `ios/AppIcon.appiconset` folder from the root of the Nit project. This is similar to the Android support where the icon files are stored in the `res` folder.

Also adds `LaunchScreen.storyboard` to the generated project. This file defines a kind of splash screen displayed while the app loads. The main advantage of this file is that it fixes a bug on physical devices where black bars were visible at the top and bottom of the screen.

The launch screen shows the app name in the center with _app.nit_ at the bottom. This is the default format provided by Xcode but it does not follow the official guidelines. In the future we may want an option for the user to provide a custom file.

To generete these icons, this PR update `svg_to_icons` with support for the icon formats expected by Xcode.

As examples and test, both Tnitter and calculator are updated with icons for iOS.

Pull-Request: #1949
Reviewed-by: Jean Privat <jean@pryen.org>