Ctrl + +/- changes point size, not line width

### 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

Ctrl + +/- changes point size, not line width

Ubuntu 20.04, Open3d 0.15.2

### Steps to reproduce the bug

```python
# Render a line set using o3d.geometry.LineSet and open3d.visualization.draw_geometries
```

### 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: 0.15.2
- System architecture: x86
- Is this a remote workstation?: no
- How did you install Open3D?: pip
```

### Additional information

_No response_