Enable keyboard control in WebRTC

### 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

When trying to run all-widgets.py with enabled WEBRTC. It is not possible to edit values which are editable without WEBRTC.
I have this problem with Python 3.9 on Debian 11 with open3d 0.14
Same behavior Windows 10 Python 3.8 open3d 0.13

Is there something wrong I am expecting?

```python
def main():
# We need to initalize the application, which finds the necessary shaders for
# rendering and prepares the cross-platform window abstraction.
_**o3d.visualization.webrtc_server.enable_webrtc()**_
gui.Application.instance.initialize()

w = ExampleWindow()

# Run the event loop. This will not return until the last window is closed.
gui.Application.instance.run()

if __name__ == "__main__":
main()
```