Allow to view std::string in debugger as utf-8 string (now it's displayed only as "latin-1")

Consider following example:

#include <string>

int main()
{
   std::string str = "тест"; // some utf-8 text

   return 0;
}

"str" string in debugger can be viewed only as latin-1 string.
For "char *" strings it is possible to select character encoding for text (at least between some different utf-* encodings), I suggest to implement displaying std::string strings in different encodings (at least in utf-8).