Adds a style class to this, so later uses of the style context will make use of this new class for styling.
In the CSS file format, a `GtkEntry` defining a “search” class, would be matched by:
```css entry.search { ... } ```
While any widget defining a “search” class would be matched by: ```css .search { ... } ```
| this |
a `GtkStyleContext` |
| class_name |
class name to use in styling |