Css for the ontology views
Show "used" predicates in the ontology

* this is a first (naive) implementation to allow the usage of
  a subset of the (600+) predicate list from Schema.org
* improvements needed:
  * this needs to go in dbd_onto_engine
    (but that is non-trivial, since the short-cut in dbd_onto
     with a class instance variable makes it difficult to
     add facts to it reliably (would be modifying the single
     master copy of the schema.org predicates)
  * when inside dbd_onto, at least use a separate CSV file
    (this could then be modified reasonably safe). Of course,
    the timing order of the facts is then skewed (these additional
    usage facts may need to be "old" to keep the order correct).