Sets the `GMenuModel` from which the popup will be constructed.
menu_model is null, the button is disabled.
A [class@Gtk.Popover] will be created from the menu model with [ctor@Gtk.PopoverMenu.new_from_model]. Actions will be connected as documented for this function.
If [property@Gtk.MenuButton:popover] is already set, it will be dissociated from the this, and the property is set to null.
a `GMenuModel`, or null to unset and disable the button