reworked variant of john’s soundfile locale fix

reworked variant of john’s soundfile locale fix

see 87b89a6

IMPORTANT NOTE: In theory, the correct glibmm function should have been Glib::filename_from_utf8() but I couldn't make that work on Windows and
ended up using Glib::locale_from_utf8() instead. sfdb import will therefore
need to get re-tested on the other platforms (especially in a non-English locale).

If this fix doesn't work we should probably revert to the previous strategy
but using the global specifier "::g_open()" explicitly…
… and only on PLATFORM_WINDOWS  (POSIX #define g_open open) fails regardless.