GDK: Prefix key names with KEY_

The keysyms create a lot of potential namespace conflicts for
C, and are especially problematic for introspection, where we take
constants into the namespace, so GDK_Display conflicts with GdkDisplay.

For C application compatiblity, add gdkkeysyms-compat.h which uses
the old names.

Just one user in GTK+ continues to use gdkkeysyms-compat.h, which is
the gtkimcontextsimple.c, since porting that requires porting more
custom Perl code.

demos/gtk-demo/hypertext.c

@@ -113,8 +113,8 @@ key_press_event (GtkWidget *text_view,
 
   switch (event->keyval)
     {
-      case GDK_Return: 
-      case GDK_KP_Enter:
+      case GDK_KEY_Return: 
+      case GDK_KEY_KP_Enter:
         buffer = gtk_text_view_get_buffer (GTK_TEXT_VIEW (text_view));
         gtk_text_buffer_get_iter_at_mark (buffer, &iter, 
                                           gtk_text_buffer_get_insert (buffer));