Merge: iOS: complete support for the UI API

Merge: iOS: complete support for the UI API

This PR complete the iOS support of the _app.nit_ UI API and fixes a few bugs:

* Fix `ListLayout` (the scrolling view) so its content fits in the horizontal limits of the screen. (The solution was the "-0-" in the constraint string!)
* Fix detecting when the user taps the system "back" button to correctly resume the previous window.
* Implement `size=` and `align=` on `TextInput` and `Button`. There is some code duplication because the Objective-C properties are not the same.
* Add `TextInput::placeholder=`, the placeholder is often used to identify a text field without using a label on iOS.
* Intro more color related services to extern classes, including a constructor to get the default light gray color used in the background of system menus.
* Bypass the Unicode validity checks so that Unicode characters are left as is, a workaround for #1945. This will cause problems on invalid Unicode strings, but it does work for valid Unicode strings.
* As a bonus, update the resolutions of the icons generated by `inkscape_tools`. Dropping support for older iOS versions and adding support for newer versions.

Pull-Request: #2320
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>