implement XDGPaths.resolveFile

[karolherbst]

currently this method always returns the default path without searching if the file actually exists

• [ ] implement XDGPaths.resolveFile(const char * singlePath, const boost::filesystem::path& singleDefault, std::string fileName)
• [ ] implement XDGPaths::resolveFile(const char * singlePath, const boost::filesystem::path& singleDefault, const char * multiPath, const char * multiDefaults, std::string fileName)
• [ ] write unit test cases