Calculator refactor & numerics proposition

* Modularize calculator.nit into 3 modules and move to a project structure.
* Rewrite the code behind the displayed string.
* Use numerics to switch nicely between `Float` and `Int`
* No more weird bug with the C button.

The Android UI will follow in another PR.
test échoué car `nitg` n'est pas un lien relatif ici. `../../bin/nitg` devrait marcher mieux