introspection: remove --allow-unprefixed

Remove --allow-unprefixed option to the scanner, and fix resulting
problems:

 * theme.h and boxes.h are split into a main -header and a private
   header that includes stuff that is not generally useful and
   hard to introspect. Merge theme-parser.h into theme.h.

 * meta_display_get_atom() and meta_window_get_window_type_atom()
   are marked as (skip)

 * Fix annotation: (element-type Strut) => (element-type Meta.Strut)

https://bugzilla.gnome.org/show_bug.cgi?id=632494

src/ui/theme-viewer.c

@@ -24,7 +24,6 @@
 #include <config.h>
 #include "util.h"
 #include "theme.h"
-#include "theme-parser.h"
 #include "preview-widget.h"
 #include <gtk/gtk.h>
 #include <time.h>