Allow to query multiple endpoints

I think there is a logical error in putting all the queries in the filter box: the first query in your demo was something like a filter, but the second query was conceptually more of a field or a relationship thing - it adds data to the object you'll be able to output.

I think it would be best to add a panel for resources to views. In the resources panel you could define a sparql endpoint and the sparql query you want to run on it. Once you added a resource it would become available to the other panels as filters, fields, relationships (to jump 2 triples far), arguments, etc (much like you do now, except that you define your resources in your filter)

Right now you are using filters to add data while they are conceptually for removing items.

Congrats again with the project, great to see the progress!