Implementing mouse-events for images

### 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

Hello!

I'm sorry if I somehow missed how this could be solved elsewhere, but I was trying to view and interact with a point cloud and an image at the same time, specifically I want to select points in the image, and can't find anything similar to the `set_on_mouse` for
`SceneWidget`s function for `ImageWidget`s.

Could someone either help me work around this or help me figure out how I can implement this myself?

Martin