This change makes it possible for GtkDialog to pack
its action widgets into a header bar, instead of the
traditional action area. This change is controlled
by the use-header-bar construct-only property.
https://bugzilla.gnome.org/show_bug.cgi?id=720059
As the first composite widget in GTK+, this patch also
adds some Makefile mechanics to list the ui files as
dependencies of the global GTK+ resources, and adds the
initial test case where composite classes should be tested.
