[xml] Initial implementation of redefinitions for the schema processor

[Kasimier Buchcik <K Buchcik 4commerce de>]

Hi,

We committed an initial support for "redefinitions" into
the schema processor. Unfortunately, in order to implement
redefinitions, we had to restructure most of the code
which handles attribute declarations and attribute group
definitions; so new bugs might have sneaked in, although
the regression tests and the W3C test suite do not show any
new errors.
Redefinitions are not well tested though, and we'll be happy
about anyone who can provide test cases or is eager to find
bugs. The W3C test suite has a few <redefine> test, but I
did not went through them yet, and since this is a wobbly part
of the spec, those tests might be useless.

There was an important fix regarding default/fixed attributes,
thus you might want to use the latest CVS revisions if you
encounter problems.

Have fun with it.

Regards,

Kasimier