- MethodChannel is now handled inside the controller
- Removed controller init, pass globalKey as required parameter
- Controller now returns a stream of data
- Updated README.md + example

- the QrViewController has a getter that returns a Stream of data that gets updated on every scan result
  - theres no need to init the QrViewController through a method, because the GlobalKey is generated inside the QrViewWidget
  - a dispose method was added to correctly dispose the stream