How to "implement" vertex selection 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

Hi,
I would like to have the same functionality as `VisualizerWithVertexSelection` but using GUI classes.
Is it possible to somehow alter the implementation shown in the example: [examples/python/visualization/mouse_and_point_coord.py](https://github.com/isl-org/Open3D/blob/master/examples/python/visualization/mouse_and_point_coord.py) such that it returns the selected vertex on the mesh?
I have tried using the naive approach by finding the closest vertex to the point that's obtained as coordinates `world`, but somehow I always get the wrong point. Moreover, even though I click on the mesh, I never get the point that's contained in the vertices set of the mesh.