Idle playback in GUI

### 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).

### My Question

I want to be able to press spacebar and run a playback function that idly animates the main 3D scene.

I'm not sure how to do this; seems related to the idea of an animation playback: https://github.com/isl-org/Open3D/issues/2869

Start by reading the related animation playback issue #2869 and the Open3D GUI context described here. Determine how idle animation of the main 3D scene and spacebar playback should work; done means the requested playback behavior is implemented and usable from the GUI.

Written by the indexing model from the issue text.