Moved the size listener to plugin side.

- Added optional parameter overlay to QrCodeScanner.
  - If overlay != null, a shapeBorder gets added on top of the scanner view.
  - Added class QrScannerOverlayShape, which is a customizable overlay.
  - updated example to use an overlay.

- 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