Merge: app.nit: Intro the `app_files` annotation, and use it in calculator

Merge: app.nit: Intro the `app_files` annotation, and use it in calculator

Intro `app_files`, an annotation to specify where to find more Android and iOS specific files. This effectively associates resources to a specific Nit module. It can be used to have different icons per branding, or to package graphical assets with a module from the lib.

It is used in the calculator to assign a different icon to the scientific calculator variant.

Pull-Request: #2228
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Romain Chanoir <romain.chanoir@viacesi.fr>