Smallest Flutter application

The small following example contains already multiple Flutter/Dart and OOP concepts.
I'll try to explain them which hopefully will allow us to fully understand how this application works.

```dart
import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
@override
Widget build(BuildContext context) {

return Center( child: Text('hello', textDirection: TextDirection.ltr)
);
}
}
```
- [x] how to import packages
- [x] the main method https://github.com/dwyl/learn-dart/issues/6#issuecomment-734336778
- [x] arrow function see https://github.com/dwyl/learn-dart/issues/6#issuecomment-734259680
- [x] the runApp method https://github.com/dwyl/learn-flutter/issues/63#issuecomment-737384276
- [x] method with named parameters https://github.com/dwyl/learn-dart/pull/20/commits/47e2f03e8d9cdc9650749cd1fa813fbd738f0ef3
- [x] abstract class using extends see https://github.com/dwyl/learn-dart/pull/20/commits/9ac9d45c7c47c1768467da5db7ef84e5b9ed1f0d for all the oop concepts checkbox below
- [x] class interface using implements
- [x] how inheritance works with Dart
- [x] How to create a **new** instance of a class
- [x] MyApp widget https://github.com/dwyl/learn-flutter/issues/63#issuecomment-738150331
- [x] stateless widget https://github.com/dwyl/learn-flutter/issues/63#issuecomment-738150331
- [x] override annotation
- [x] buildContext https://github.com/dwyl/learn-flutter/issues/63#issuecomment-738150331
- [x] widgets composition
- [x] single vs double quotes? see https://github.com/dwyl/learn-dart/issues/19