Find better intermediate format solution for TOCs
Currently every tei:zone[@type = 'toc'] is transformed to a tei:list in the intermediate format.
This doesn't consider the encoding, in which we sometimes have text that isn't part of an item, but some kind of introduction to the TOC, e.g.
<zone type="toc" subtype="Friedrich_Fontane">
<zone><line>Inhalt:</line>
<zone type="item"><line>Enthält Entwürfe zu</line></zone>
</zone>