Merge: app.nit on Android: implement multiple windows support with fragments

Merge: app.nit on Android: implement multiple windows support with fragments

This PR intro support for Andorid apps composed of multiple windows. There is now one fragment par app.nit window, and it is set as visible when assigned to `app.window=`.

Support for multi-windows apps on other platforms will be provided by future PRs, along with a portable API for the back button and to navigate between windows. This system, on Android at least, could probably be extended to show more that one window at a time on larger tablet devices.

Pull-Request: #1989
Reviewed-by: Jean Privat <jean@pryen.org>