Implement gtk_settings_get_default
Implement a Settings to GObject Converter

This allows for GObject methods to be called on Settings objects.

In C:
	GtkSettings *default_settings = gtk_settings_get_default();
	g_object_set(default_settings, "gtk-button-images", TRUE, NULL);

In Go:
	var settings *glib.GObject = gtk.SettingsGetDefault().ToGObject()
	settings.Set("gtk-button-images", true)