don't permit using CEF builds that will have encoding problems with slot saves; we assume UTF16 strings.

Be a bit stricter about the encoding used for slot saves to avoid slot corruption with certain high-unicode characters, such as .

Consolidate the general, platform-agnostic part of the main()/WinMain()
in static FCHost::main() function to avoid code duplication.

Linux handlers and main() were copied from the cefsimple test app,
persistent storage implementation rewritten using cstdio and
std::filesystem to make it cross-platform.