CharsetConverter: add extra care on using wchar_t as UTF-32
* check for __STDC_ISO_10646__
* copy-convert only for UTF-32 -> wchar_t, but use converter for wchar_t -> UTF-32 (UTF-32 has more restrictions)
CharsetConverter: use UTF-32 charset with correct endianness
Merge pull request #3353 from Karlson2k/charsetconverter_fix_02_alt2

CharsetConverter UTF-32 fix