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

[caltaluc]

Checklist

• [X] I have searched for similar issues.
• [X] For Python issues, I have tested with the latest development wheel.
• [X] I have checked the release documentation and the latest documentation (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

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

- 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