Wrap GtkSettings new properties as of 3.12
Wrap GtkSettings new properties as of 3.12

* GtkSettings::gtk-decoration-layout property
* GtkSettings::gtk-shell-shows-desktop property
* GtkSettings::gtk-dialogs-use-header property

GNOME/gtkmm,johne53/MB3Gtkmm,GNOME/gtkmm,johne53/MB3Gtkmm,GNOME/gtkmm,johne53/MB3Gtkmm,GNOME/gtkmm