How to adjust text size when using gui.Label() method?

### 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).

### My Question

My request is that draw a window and show some text info at the left-top corner of the window, and I use **gui.Label('label_name')** to realize this.
But the **text size is too small for the whole window**, I checked the document about gui.Label and search the issues and then found that there was no method to adjust text size directly (but there was a method about gui.label3D, but this method not fit my request), so I want to ask is there any method to adjust label text size?

Below is the pseudo-code

```import open3d as o3d
import open3d.visualization.gui as gui
import open3d.visualization.rendering as rendering

gui.Application.instance.initialize()
self.window = gui.Application.instance.create_window('Example', width=1200, height=800)
self.widget = gui.SceneWidget()
self.widget.scene = rendering.Open3DScene(self.window.renderer)
self.widget.scene.set_background([0, 0, 0, 1])
self.window.add_child(self.widget)

em = self.window.theme.font_size
self._timestamp_text = gui.Label('TimeStamp')
self._timestamp_text.text_color = gui.Color(0.0, 1.0, 0.0)
horizon_layout = gui.Horiz()
horizon_layout.add_child(self._timestamp_text)
vertical_layout = gui.Vert()
vertical_layout.add_fixed(0.5 *em)
vertical_layout.add_child(horizon_layout)
self.window.add_child(vertical_layout)
```

And the visualization result like:

It seems that text info font size is too small relative to thole window.

Start with the gui.Label documentation and the shown use of window.theme.font_size. Trace the GUI label and theme entry points to determine whether label-specific sizing is supported. Done means providing or documenting a way to make gui.Label text larger than the current window theme size, with suitable coverage for the Python API.

Written by the indexing model from the issue text.