How can I get the point cloud selected by my mouse in VisualizerWithEditing?

### 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 know that in VisualizerWithEditing I can use the mouse to select directly, and use the keyboard "C" or "S" to save the selected point cloud. But I want to know whether I can obtain the information of the point cloud in the current vis through some interfaces? Instead of saving and reading file.