Avoid some allocations by QString/QByteArray's toLower()
No benchmarking done, based on heuristic tales that memory allocation is to be avoided where possible.
Not exactly sure how often a conversion actually happens, even more when Qt's algorihtms seem smart enough these days to only do a deep copy when actually needed. But it matches existing pessimistic code which is ready to deal with manually-crafted config with non-standard casing.