Merge: iOS: implement data_store and missing life cycle callbacks

Merge: iOS: implement data_store and missing life cycle callbacks

Implement the main missing features of _app.nit_ on iOS: `data_store` and life-cycle hooks (like `on_save_state`). These changes can be seen on the calculator app, as it preserves its context using the `data_store`.

`data_store` is implemented with `NSUserDefaults` to store objects sertialized to Json. It is very similar to Android's implementation using shared preferences. This may be a bit limited as it is not meant to hold large strings, and some data objects (like game saves) should instead be saved to a file.

This PR also implements all life-cycle callbacks in iOS, until now only `on_create` was implemented. We may have to update _app.nit_ life-cycle to fit better with the life-cycle of iOS, the states between a running app and a fully stopped app are different between iOS and Android. I'm thinking of removing the two callbacks on_start/on_stop and keep only the more general callbacks on_create/on_destroy and on_resume/on_pause, and the services on_restore_state/on_save_state.

Pull-Request: #1947
Reviewed-by: Jean Privat <jean@pryen.org>