Replace all occurrences of std::string with CPLString

[Mateusz Łoskot]

In some files, there is still std::string class used directly instead of CPLString​ wrapper. This task suggests to review and replace std::string with CPLString.

[Mateusz Łoskot]

I'm closing my own ticket re-reviewing it. In my opinion, it should be changed the other way around, replace all CPLString with std::string. The CPLString is generally a bad idea declaring conversion operator char* forbidden in std::string by design. The printf-like functions are a potential security backdoors allocating big chunks on stack and driving additional allocations by return value from vsnprintf which fails due to formatting failures, not only buffer sizes).