DVDSubtitleStream: better detect stream encoding and optimization
Use BOM detection for all supported charsets, not only UTF-8 and UTF-16LE
Do not double convert subtitle->wstring->UTF-8. Use direct conversion subtitle->UTF-8
additionally: CharsetConverter: subtitleCharsetToW -> subtitleCharsetToUtf8
Merge pull request #3914 from Karlson2k/wstring_replace_01

DVDSubtitleStream: better detect stream encoding and optimization