Setting a child resets [property@Gtk.MenuButton:label] and
[property@Gtk.MenuButton:icon-name].
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 child.
Sets the child widget of @menu_button.
Setting a child resets [property@Gtk.MenuButton:label] and [property@Gtk.MenuButton:icon-name].
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 child.