Annotation explorer should also show available document metadata categories

[Amir Zeldes]

The annotation explorer has three segments on the right side: node annotations, edge types and edge annotations. There should be a fourth segment giving the metadata available in any of the documents or subcorpora in the corpus (even if different documents have different metadata field names). The fourth segment should then list each category of document or subcorpus metadata with an example value (e.g. the most frequent one, just like in the node annotations segment).