When a variant has optional, it is possible to explictly list it in the
XML file and add extra groups. However, the original DTD did not allow
multiple variants with `id="optional"` as `id` attribute has to be
globally unique. This patch changes its type to `CDATA`, which has less
restrictions. This also means we can no longer define `<ref>` as
`IDREF` and instead check the existence of referenced variant in parser.
Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>