Convert macro+enum into a simple enum: Currently the enum was filled using a

Convert macro+enum into a simple enum: Currently the enum was filled using a
macro that also contained a description for every field, but, that description
was not being used anywhere. This changes it, to make the description available
as a Doxygen comment. Furthermore, it is now easily parsable by Doxygen, while
before it messed up the documentation.

Originally committed as revision 8254 to svn://svn.ffmpeg.org/ffmpeg/trunk