gtk3: unify `Gtk::Stack#set_visible_child` and `Gtk::Stack#set_visible_child_full`
gtk3: unify `Gtk::Stack#set_visible_child` and `Gtk::Stack#set_visible_child_full`

These are similar operations, so it is reasonable to unify APIs.

kitachro/ruby-gnome2,kitachro/ruby-gnome2,kitachro/ruby-gnome2,kitachro/ruby-gnome2,kitachro/ruby-gnome2