Port to GSettings

Fixes #886056. I didn't bother with the data conversion because src:gconf2 is likely to be removed during this cycle so it won't be available to users upgrading from buster to bullseye.

I'm not familiar with this game so I tested it mostly to confirm that there are no regressions and TTBOMK, there aren't. Please test extensively as my C++ skills are practically non-existent. Thanks.