CUtf8Utils: add "checkStrForUtf8()"
New UTF-8 string check, based on current Unicode standard (6.3), with three result values: plain ASCII, valid UTF-8, high ACSII.
Utf8Utils: add "isValidUtf8()" for convenient check for valid UTF-8 strings
Remove usage of CharsetConverter.isValidUtf8()
CharsetConverter: remove outdated "isValidUtf8()"
Merge pull request #3750 from Karlson2k/update_utf8_check

Update UTF-8 string check