Being able to set the open3d.visualization.gui.Combobox selected item

### Checklist

- [X] I have searched for [similar issues](https://github.com/isl-org/Open3D/issues).
- [ ] 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).

### Proposed new feature or change

It does not seem possible to define the default selected item of an open3d.visualization.gui.Combobox in python.
More generally, it does not seem possible to select an item from a gui.Combobox() by code.

Thank you for your work. I really like this library.

### References

_No response_

### Additional information

_No response_