init GtkMenuButton #112

[d47081]

Initial commit, methods not implemented yet

112

[d47081]

Just added set_popup method.

If everything implemented properly, suppose we can merge at this point, because my app requires separated fork and I don't plan to implement another methods for right now

For test:

        $button = new \GtkMenuButton;

        $menu = new \GtkMenu();

        $menu->append(
            \GtkMenuItem::new_with_label("Test 1")
        );
        $menu->append(
            \GtkMenuItem::new_with_label("Test 2")
        );

        $menu->show_all();

        $button->set_popup(
            $menu
        );

        $somegtk->add(
            $button
        );

Reference: https://docs.gtk.org/gtk3/method.MenuButton.set_popup.html