Replace window label string concatenation with interpolation

[itsrenderman]

More aesthetically pleasing and easier to read over the former concatenation

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[autonordev]

Thanks for the pull-request. While it is more aesthetically pleasing to use interpolation, the Cmdr codebase is full of legacy code that works just fine (including concatenation and non-generalised iteration on dictionaries). Accordingly, commits which exclusively replace legacy with non-legacy features aren't ideal. If you're working on a contribution and see some related legacy code, changing it to modern features is OK though. 'Aesthetically pleasing' is itself not a very compelling reason to accept a change, even a plain simple one like this, given that the current legacy code is easily readable.