Lots of changes in regards to strings, as follows:

* Removed my custom String class, as well as removed the really old
Unicode ConvertUTF, the UTF Converter, and the UTF Encode/Decode
functions.
* Replaced the above with using standard C++ std::wstring_convert and
std::codecvt_utf8.
* Added headers adapted from llvm's libc++ project for allowing the
above C++ classes to exist with GCC and Clang when libc++ isn't being
used.
* Used std::string over std::wstring whenever possible.
* Added header adapted from llvm's libc++ project to create special
versions of the ifstream and ofstream classes that would utilize _wfopen
on non-MSVC Windows compilers, so those compilers could access files on
Windows that use characters outside the current codepage.
* Removed the -static-libgcc and -static-libstdc++ flags from the
Makefile for non-MSVC builds. The generated DLLs will require extra DLLs
from either Cygwin or MinGW (whichever is used to compile), I've only
tested with MinGW and for some reason they crash Winamp on exit, I have
no idea why.