Divide categories into four groups as they are processed from the XML. Add

Divide categories into four groups as they are processed from the XML.  Add
an iterator to iterate over the categories in order, starting with "core"
versions, then ARB extensions, then numbered non-ARB extensions, and finally
unnumbered extensions.

Use the new iterator in a couple places to ensure that output that is
grouped by catgory is generated in a consistent order.

More changes to the scripts are coming.  The generated files will be
committed one time after all the changes are in.  Too bad we're not using
GIT, or this would be easy. :(