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.

gtk/gtkmain.c

@@ -1713,7 +1713,7 @@ gtk_main_do_event (GdkEvent *event)
       /* Catch alt press to enable auto-mnemonics;
        * menus are handled elsewhere
        */
-      if ((event->key.keyval == GDK_Alt_L || event->key.keyval == GDK_Alt_R) &&
+      if ((event->key.keyval == GDK_KEY_Alt_L || event->key.keyval == GDK_KEY_Alt_R) &&
           !GTK_IS_MENU_SHELL (grab_widget))
         {
           gboolean auto_mnemonics;