Feature summary (what you would like to be able to do and where):
Currently, the file default-config.json contains the config data to connect to the Wikidata endpoint.
What are the values for Wikimedia Commons Structured Data?

Also, I understand there is a Wikidata Test server. Could config for that service also exist?

Use case(s) (list the steps that you performed to discover that problem, and describe the actual underlying problem which you want to solve. Do not describe only a solution):
Any one who wants to use the GUI Query tool with Wikimedia Commons.

Benefits (why should this be implemented?):
Broader use of the tool, easier configuration.