Small test case showcasing Gobject introspection usage and some of the later features introduced in GTK (action widgets in GtkNotebook, GtkSpinner).
Some notable issues required to be resolved in Gobject introspection itself before we can consider using it:
* No default values in e.g. GtkBox.pack_start()
* Loss of convenience wrappers like GtkTreeRow for simple value access, __gproperties__, __gsignals__, …