GUI Additions to VisualizerWithVertexSelection

### 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](https://www.open3d.org/docs/latest/getting_started.html#development-version-pip).
- [X] I have checked the [release documentation](https://www.open3d.org/docs/release/) and the [latest documentation](https://www.open3d.org/docs/latest/) (for `main` branch).

### My Question

I'm trying to make an application that lets users select parts of a 3D model and designate them as having certain traits. I want a user to select the points of interest and then click a button that designates them as having a given trait. I would prefer to do this without making the user close the visualization window. I've been trying to use open3d.visualization.VisualizerWithVertexSelection for this, but I can't get it to integrate with tkinter and it doesn't seem to have the capability for adding GUI buttons to the visualization window. Is there some other solution I should try for this?