Query templates: allow to configure label vs value

[bjdmeest]

Right now in the product dropdown you have to choose between the actual values that are put in the SPARQL query. Being able to set a label would make things more user-friendly

[mvanbrab]

This implies a translation table (real template variable content) / (label to display) is added in the configuration file, which in my opinion makes things less transparent to the user.
If the query is designed to enter a URI in a variable, show URIs to the user.
If the query can be redesigned to use strings, redesign the query. This can be the case if every entity has a unique string property, as is the case with the "genre" in the templated query example.
See also #78.