Sets the name of an icon to show inside the menu button.
Setting icon name resets [[email protected]:label] and [[email protected]:child].
If [[email protected]:always-show-arrow] is set to `TRUE` and [[email protected]:direction] is not `GTK_ARROW_NONE`, a dropdown arrow will be shown next to the icon.
| this |
a `GtkMenuButton` |
| icon_name |
the icon name |