Graph Explorer Namespaces

[LeonBein]

As a developer and potentially analyzing user,
I'd like the graph explorer box to be pre-filled with the graph namespaces,
so I can write queries more easily, reusing ids from the graphviz.

Acceptance criteria:

• When writing a query in the graph exploration view, prefixes of the pkg are already pre-bound
• There is a way to see the available prefixes

Comment:
In the end, this can only be one small step toward properly supporting users with graph exploration. Solutions like, e.g., https://github.com/sparna-git/Sparnatural should be the future goal.