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.
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.