Widget using Selectable like ComboBox can not accept items with same string

### 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).

### Describe the issue

While ComboBox contains same items, only the first item could be selected. While click on the non-first item, nothing happens.

### Steps to reproduce the bug

```python
modify all_widgets.py in examples like:

class ExampleWindow:
131┊131┊ # This is a combobox, nothing fancy here, just set a simple function to
132┊132┊ # handle the user selecting an item.
133┊133┊ combo = gui.Combobox()
┊134┊ combo.add_item("data")
┊135┊ combo.add_item("data")
┊136┊ combo.add_item("data")
134┊137┊ combo.add_item("Show point labels")
135┊138┊ combo.add_item("Show point velocity")
136┊139┊ combo.add_item("Show bounding boxes")
```

run it and select 2nd or 3rd 'data', you'll find it can not be selected.
```

### Error message

_No response_

### Expected behavior

Same item should be able to selected in ComboBox

### Open3D, Python and System information

```markdown
- Operating system: Any
- Python version: Any
- Open3D version: 0.14.1
- System type: Any
- Is this remote workstation?: no
- How did you install Open3D?: build from source
- Compiler version (if built from source): gcc 9.3
```

### Additional information

This issue is important in some scenario like FileDialog, the item in dir tree can not be selected makes it fail to select target dir.