Organize xkbcomp/ header files

Organize xkbcomp/ header files

Various non-functional changes:
- Re-add keycodes.h and move some stuff there.
- Add parser-priv.h for internal bison/flex stuff.
- Don't include headers from other headers, such that file dependencies
  are immediate in each file.
- Rename xkbcomp.h -> ast.h, parseutils.{c,h} -> ast-build.{c,h}
- Rename path.{c,h} -> include.{c,h}
- Rename keytypes.c -> types.c
- Make the naming of XkbFile-related functions more consistent.
- Move xkb_map_{new,ref,unref} to map.c.
- Remove most extern keyword from function declarations, it's just
  noise (XKB_EXPORT is what's important here).
- Append XKBCOMP_ to include guards.
- Shuffle some code around to make all of this work.

Splitting this would be a headache..

Signed-off-by: Ran Benita <ran234@gmail.com>