TreeView row highlight bug.

### 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

on the gui, when you create a treeView, and add labels to it, you have to click on the font to highlight the row, and not the row itself, which makes it really hard to click on the row.

to make it more clear:
I have to click on the letters for the row to get highlighted.

### Steps to reproduce the bug

```python
_preset_pov = gui.TreeView()
_preset_pov.clear()

root_id = _preset_pov.get_root_item()
#run over a set of strings and add them to the TreeView
for name in self.configurations.pov_presets_names:
_preset_pov.add_item(root_id,gui.Label(name))
```

### Error message

_No response_

### Expected behavior

when you click on the row, the row is highlighted.

### Open3D, Python and System information

```markdown
- Operating system: Ubuntu 20.04
- Python version: Python 3.8
- Open3D version: output from python: 0.14.1
- System architecture: x86
- Is this a remote workstation?: no
- How did you install Open3D?: pip
- Compiler version (if built from source): gcc 7.5
```

### Additional information

_No response_