Add update_geometries function for gui.SceneWidget

### 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](https://www.open3d.org/docs/latest/getting_started.html#development-version-pip).
- [X] I have checked the [release documentation](https://www.open3d.org/docs/release/) and the [latest documentation](https://www.open3d.org/docs/latest/) (for `main` branch).

### Proposed new feature or change

Could the ability to update geometries please be added to the gui.SceneWidget? At the moment it appears that the work around is to remove_geometry then add it back with add_geometry. However this takes a bit of time to complete (with larger point clouds).

This issue has been referenced before (#issue_6430) , but has been marked as resolved - however I believe the current fix is just temporary. As, In the [Example](https://github.com/isl-org/Open3D/blob/main/examples/python/visualization/online_processing.py) , there is a comment on line 379:

``` TODO(ssheorey) Switch to update_geometry() after #3452 is fixed. ```

Thanks.

### References

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### Additional information

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