Drop explicit includes of gdkkeysyms.h

These are no longer needed. At the same time, port gtkimcontextsimpleseqs.h
to use the new GDK_KEY_ symbols.

gtk/gtkexpander.c

@@ -22,7 +22,9 @@
  */
 
 #include "config.h"
+
 #include <string.h>
+
 #include "gtkexpander.h"
 
 #include "gtklabel.h"
@@ -32,7 +34,6 @@
 #include "gtkmain.h"
 #include "gtkintl.h"
 #include "gtkprivate.h"
-#include <gdk/gdkkeysyms.h>
 #include "gtkdnd.h"