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. In GTK2, this header is included by default.
https://bugzilla.gnome.org/show_bug.cgi?id=629093
