More changes to the custom attribute mechanism:

More changes to the custom attribute mechanism:

o  Introduced the concept of extended attributes which consist of the
   custom attributes and some internally defined attributes handled
   in the same way.
o  FILETYPE and LANG are now extended attributes and they don't depend
   on the customization module. This is probably much cleaner as it
   reduces the overlap between custom parts and generic parts.
o  No problems with linking liblang because of LANG attribute.
o  idxdump got simplified.