Drag Window Event for open3d.visualization.gui.Window

### 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 created a Window with the open3d gui module. I have a PYQt Overlay that I want to move at the same time as the window.

I checked the on_tick event but it isn't called during dragging of the window. It pauses for the duration.
Also the event.MOVE or event.DRAG are not triggered outside of the SceneWidget.

Do you have any suggestions?