fix: truncating chapter titles using UTF-8 safe function (#599)

## Summary

* Truncating chapter titles using utf8 safe functions (Cyrillic titles
were split mid codepoint)
* refactoring of lib/Utf8

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src/util/StringUtils.h

@@ -19,10 +19,4 @@ std::string sanitizeFilename(const std::string& name, size_t maxLength = 100);
 bool checkFileExtension(const std::string& fileName, const char* extension);
 bool checkFileExtension(const String& fileName, const char* extension);
 
-// UTF-8 safe string truncation - removes one character from the end
-// Returns the new size after removing one UTF-8 character
-size_t utf8RemoveLastChar(std::string& str);
-
-// Truncate string by removing N UTF-8 characters from the end
-void utf8TruncateChars(std::string& str, size_t numChars);
 }  // namespace StringUtils