After this change, we’re managing Counter’s lifecycle with ChangeNotifierProvider.
This removes the periodic Timer. Not only does that simplify the example and makes it closer to the original, it also prevents a leaking timer (though, in this case, it’s not an issue, since the timer is needed for the whole duration of the app). I experimented with a more robust approach (having or injecting a Timer/StreamController into the Counter, and disposing of it there) but that seemed overly complex for such a simple example. This whole problem will get significantly easier with https://github.com/rrousselGit/provider/issues/46, at which point I could reintroduce this. I will also think about a more complex Provider example, something like the venerable `bloc_complex`, with infinite-scrolling pagination and all that.
I used `flutter create` anew, so the project structure now has additional 2 `AndroidManifest.xml` files. Everything else stayed the same, so from git’s perspective, it’s just moved files.
