Add a gtkx.h header for X11-specific GTK+ api

This keeps us from pulling gdkx.h into gtk.h

https://bugzilla.gnome.org/show_bug.cgi?id=640902

docs/reference/gtk/gtk3.types

@@ -1,4 +1,4 @@
-#include <gtk/gtk.h>
+#include <gtk/gtkx.h>
 #include <gtk/gtkunixprint.h>
 
 gtk_about_dialog_get_type

docs/reference/gtk/x11.sgml

@@ -31,6 +31,13 @@ see <link linkend="gdk-X-Window-System-Interaction">GDK X Window
 System interaction</link> in the GDK manual.
 </para>
 
+<para>
+GTK+ includes an cross-process embedding facility in the form of
+the #GtkSocket and #GtkPlug widgets. These are X11-specific, and
+you have to include the <filename>gtk/gtkx.h</filename> header
+to use them.
+</para>
+
 <refsect2 id="x11-cmdline">
 <title>X11-specific commandline options</title>