setIconName  
  Sets the name of an icon to show inside the menu button.
Setting icon name resets property@Gtk.MenuButton:label and property@Gtk.MenuButton:child.
If property@Gtk.MenuButton:always-show-arrow is set to TRUE and property@Gtk.MenuButton:direction is not GTK_ARROW_NONE, a dropdown arrow will be shown next to the icon.
Parameters
iconName 
the icon name