gtk3: implement Gtk::Stack

GtkStack is provided in gtk3+ 3.10 or later. see: https://developer.gnome.org/gtk3/3.10/GtkStack.html
gtk3: return self instead of Qnil.

Because of Ruby-GNOME2 naming/conversion rules.
gtk3: remove needless `set_` prefix

target function doesn't have `set_`.
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.
gtk3 test: more meaningful test case

avoid to use assert_nothing_raised.
gtk3: implement generic Gtk::Stack#add
gtk3 test: use generic Gtk::Stack#add
gtk3: remove needless `Gtk::Stack#add_named` and `Gtk::Stack#add_titled`
gtk3: use assert_false instead of assert_equal
gtk3 test: remove needless block arguments
Merge pull request #223 from cosmo0920/implement-gtk-stack

Implement gtk 3.10 feature: Gtk::Stack