Sets the direction in which the popup will be popped up.
If the button is automatically populated with an arrow icon, its direction will be changed to match.
If the does not fit in the available space in the given direction, GTK will its best to keep it inside the screen and fully visible.
If you pass gtk_arrow_none for a direction, the popup will behave as if you passed
gtk_arrow_down (although you won’t see any arrows).
| this |
a `GtkMenuButton` |
| direction |
a `GtkArrowType` |