Not able to copy/paste from system clipboard into gui.TextEdit

### 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).

### Describe the issue

As mentioned in the title, it seems that is not possible to access the system clipboard to copy/paste text into a TextEdit widget.
Is that an expected behavior or a bug? Thanks for any help!

### Steps to reproduce the bug

```python
import open3d.visualization.gui as gui

def main():
app = gui.Application.instance
app.initialize()

w = app.create_window("Copy/Paste TextEdit", 1024, 256)
textbox = gui.TextEdit()
textbox.placeholder_text = "Try to paste text here"
w.add_child(textbox)

app.run()

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

### Error message

_No response_

### Expected behavior

_No response_

### Open3D, Python and System information

```markdown
- Operating system: Ubuntu 20.04
- Python version: Python 3.8
- Open3D version: output from python: 0.18.0
- System architecture: x86
- Is this a remote workstation?: no
- How did you install Open3D?: pip
```

### Additional information

_No response_