New file collecting the type constants, includes and the union for the

New file collecting the type constants, includes and the union for the

* babl/babl-classes.h: New file collecting the type constants,
includes and the union for the babl classes. When adding a new
class this is the only installed header file that requires
modification.

* babl/babl-class.h: Move the class type enums to the new file.

* babl/babl.h: Include babl-classes.h instead of each class on its
own, and move the Babl union to the new file.

* babl/Makefile.am: Add the new file.

svn path=/trunk/; revision=385