Allow keywords in textClass

[burki]

Currently, /TEI/teiHeader/profileDesc/textClass only allows the classCode tag mainly used for the DTA-specific classification of the Genre and a DWDS-specific classification (http://www.deutschestextarchiv.de/doku/klassifikation).

In order to use different classification schemes such as GND or LCSH, adding keywords / term as in https://www.tei-c.org/release/doc/tei-p5-doc/en/html/ref-keywords.html might be helpful:

    <textClass>
      <keywords>
        <term ref="http://d-nb.info/gnd/4015656-4">Eugenik</term>
      </keywords>
    </textClass>

With classCode only, due to the lack of att.canonical (key, ref) on this element, it seems hard to record both the URI as well as the label without introducing project specific conventions such as for example <classCode scheme="...">Eugenik [http://d-nb.info/gnd/4015656-4]</classCode