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/gtkimcontextsimple.c

@@ -21,7 +21,7 @@
 #include <stdlib.h>
 #include <string.h>
 
-#include <gdk/gdkkeysyms.h>
+#include <gdk/gdkkeysyms-compat.h>
 #include "gtkprivate.h"
 #include "gtkaccelgroup.h"
 #include "gtkimcontextsimple.h"