Simple login screen.

Just validate email and password.

Not logging in. No authentication involved. Just plain client validation.

Concepts Introduced

Dart Mixins

Dart getters

Introducing Streams

- StreamBuilder
- StreamTransformer
- StreamController

Bloc pattern

-> Single global instance
-> Scoped instance
    -> Inherited Widgets

-> Use RxDart to combine email and password streams, to enable/disable the submit button.

RxDart

-> Help us work with streams.

Stream           ====> Observable
StreamController ====> Subject