Merge: Android UI, notifications, toasts and calculator

This is the base of a the API for native controls on Android. As such, is it a work in progress.

Example applications:
* app.nit UI test
* Calculator

Features still to do:
* Wrap `ViewGroup`s with Nity classes. This will allow us to make all native classes private.
* Implement `ListView` and other useful views.
* Offer more notifications features from later APIs.

Pull-Request: #934
Reviewed-by: Jean Privat <jean@pryen.org>