CharsetDetection: add ConvertHtmlToUtf8() and helper functions
ConvertHtmlToUtf8() detects HTML charset and convert HTML to UTF-8
CharsetConverter: add systemToUtf8
CharsetDetection: add "ConvertPlainTextToUtf8()"
FileUtils: add file type detection based on Mime-type
ScraperUrl: detect and use charset even if web server don't report it
Merge pull request #3796 from Karlson2k/html_charset_detecton_01

Properly detect HTML charset even if not reported by server