Add provenance information to the autocompletion results

[hdelva]

For the sake of user experience, it would be nice if each result could also explain in which data source it was found. There are several ways we could implement this:

• Comunica might add something similar for graph-based querying soon, which we may be able to reuse
• It is relatively easy to remember on which datapage the result was found, we could add some reasoning to tie this data page back to a data source. We could use the tree:member or tree:view predicates to tie this all together.
• It is perhaps unlikely that the previous approaches can extract enough human-friendly information to be of any use. We could offset this responsibility to the caller of the typeahead actor. We already consider having one instance of the actor per data source, in separate webworkers, as best practice.