In-place UTF-8 case-insensitive comparisons

I think I weeded out the last code that converted from one encoding to another just to compare strings case-sensitively.

The places where we still do that are case-insensitive comparisons involving UTF-8.

It seems it would be pretty easy to extend QUtf8::compareUtf8() with case-insensitivity, because it already compares characters at the char32_t level.

Unfortunately, qstringconverter.cpp doesn't have access to foldCase (in the qstring.cpp TU). It seems the general approach to this is to #include the .cpp file into qstring.cpp (cf. qstringmatcher.cpp, qchar.cpp), so maybe that works here, too?