Fix regression in mutter-wayland translations

Switching meta/util.h to gi18n.h was wrong, mutter is a library
and needs gi18n-lib.h, but that cannot be included from a public
header (since it depends on config.h or command line options),
so split util.h into a public and a private part.

https://bugzilla.gnome.org/show_bug.cgi?id=707897

src/compositor/plugins/default.c

@@ -21,15 +21,14 @@
  * 02111-1307, USA.
  */
 
+#include <config.h>
+
 #include <meta/meta-plugin.h>
 #include <meta/window.h>
-#include <meta/util.h>
 #include <meta/meta-background-group.h>
 #include <meta/meta-background-actor.h>
-
-#include <libintl.h>
-#define _(x) dgettext (GETTEXT_PACKAGE, x)
-#define N_(x) x
+#include <meta/util.h>
+#include <glib/gi18n-lib.h>
 
 #include <clutter/clutter.h>
 #include <gmodule.h>