How to add MouseCallback on gui.Vert ?

### 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

When changing the color of points via gui.ColorEdit, I only wants to re-render after the user's mouse is up (becasue re-rendering is such a heavy job). However I don't know how to listen to such event in gui.ColorEdit or gui.Vert