Selecting Point of a point cloud using SceneWidget

### Checklist

- [X] I have searched for [similar issues](https://github.com/isl-org/Open3D/issues).
- [X] For Python issues, I have tested with the [latest development wheel](http://www.open3d.org/docs/latest/getting_started.html#development-version-pip).
- [X] I have checked the [release documentation](http://www.open3d.org/docs/release/) and the [latest documentation](http://www.open3d.org/docs/latest/) (for `master` branch).

### My Question

i am using SceneWidget and open3dScene to display my Pointcloud, i would like to be able to click on a point that is displayed and get its coordinates, any ideas? I would like to build sth like the Selection function implemented in open3d.visualization.draw.