The keysyms defines in clutter-keysyms.h are generated from the X11 key
symbols headers by doing the equivalent of a pass of sed from XK_* to
CLUTTER_*. This might lead to namespace collisions, down the road.
Instead, we should use the CLUTTER_KEY_* namespace.

This commit includes the script, taken from GDK, that parses the X11
key symbols and generates two headers:

  - clutter-keysyms.h: the default included header, with CLUTTER_KEY_*
  - clutter-keysyms-compat.h: the compatibility header, with CLUTTER_*

The compat.h header file is included if CLUTTER_DISABLE_DEPRECATED is
not defined - essentially deprecating all the old key symbols.

This does not change any ABI and, assuming that an application or
library is not compiling with CLUTTER_DISABLE_DEPRECATED, the source
compatibility is still guaranteed.