New resource visibility handling.

Until now the query api would use the nao:userVisible values stored for
each resource to determine if a resource should be returned.
This requires a lot of nao:userVisible value maintenance and makes for
slower queries.
The new solution is much simpler: only types have nao:userVisible values
and the query api simply checks if the result candidate has a type
which is visible.
In contrast to the old one the new solution does not allow to hide a
sub-class of a visible class. But since we never did that anyway and
the resulting queries are faster and it allows us to remove a lot
maintenance code (which eats CPU cycles and disk space) the change is
worth it.