Flutter example app

[greenrobot]

The typical ObjectBox example is a simple note taking app. So let's do that using Flutter...

Destination: this repo inside a new examples/flutter/ dir.

For reference:
https://github.com/objectbox/objectbox-examples

[vaind]

Preferably, this should be done as a mobile app, because the current example uses linux-app which are not yet generally available in flutter

[vaind]

Implemented and merged onto dev: example/flutter/objectbox_demo