multiple visualization window at once

### 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 was just wondering if there is a way to have multiple visualization window at once for class `o3d.visualization.VisualizerWithEditing`. I am trying to implement [manual registration](http://www.open3d.org/docs/latest/tutorial/visualization/interactive_visualization.html#manual-registration) by picking point (correspondences) between two point cloud. Since I have not found a way to refer back to where the points were picked in source point cloud such that corresponding points could be picked in target point cloud. I thought it would be easier if two point cloud window were side by side for ease.

I went through `open3d.visualization.gui` and [multi_window.py](http://www.open3d.org/docs/latest/python_example/visualization/index.html#multiple-windows-py), but was unable to pick point in those visualization. Thank you in advance.