How can I control individual point cloud sizes with the O3DVisualizer?

### 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've seen this related issue https://github.com/isl-org/Open3D/issues/2135
But I want the menu I get with the O3DVisualizer to interactively show/hide pointclouds and groups. The example in that issue uses SceneWidget and I could't get the same menu to show? is it possible?

I adapted the example in that issue like this:

```
#!/usr/bin/env python

import numpy as np
import open3d as o3d
import open3d.visualization.gui as gui

RED = [1.0, 0.0, 0.0, 1.0]
GREEN = [0.0, 0.5, 0.0, 1.0] # dark green is easier to see on white when small
BLUE = [0.0, 0.0, 1.0, 1.0]

def add_cloud(scene, name, npts, center, radius, color, size):
pts = np.random.uniform(-radius, radius, size=[npts, 3]) + center
cloud = o3d.geometry.PointCloud()
cloud.points = o3d.utility.Vector3dVector(pts)

material = o3d.visualization.rendering.MaterialRecord()
material.shader = "defaultUnlit"
material.base_color = color
material.point_size = size
scene.add_geometry(name, cloud, material)

def main():
app = gui.Application.instance
app.initialize()
vis = o3d.visualization.O3DVisualizer("Open3D", 1024, 768)
vis.show_settings = False

add_cloud(vis, "green", 5000, [0, 0, 0], 10.0, GREEN, 2)
add_cloud(vis, "red", 500, [-2, -2, -2], 5.0, RED, 4)
add_cloud(vis, "blue", 250, [1, 1, 1], 5.0, BLUE, 20)

vis.reset_camera_to_default()
vis.show_skybox(False)
app.add_window(vis)

app.run()

if __name__ == "__main__":
main()
```

This shows all the point clouds with the last size 20. Is there any way of controlling the individual pointcloud sizes with the O3DVisualizer?