Remove Sphere, which is added after selecting a point using open3d.visualization.draw

### 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 would like to either remove or make the sphere which is automatically added after selecting a point using open3d.visualization.draw significantly smaller. The added sphere is too big as it makes me stop seeing other points.

Also, I was wondering if there is a way of implementing selecting a point and getting its coordinates and index using open3d.visualization.gui.