CsvImportAssistant: resolve clang-tidy warning about int conversions

/G/sw/ba/GUI/Views/ImportDataWidgets/CsvImportAssistant/CsvImportAssistant.cpp:329:29: warning: 'signed char' to 'int' conversion in array subscript; consider casting to 'unsigned char' first. [bugprone-signed-char-misuse,cert-str34-c]
            frequencies[int(c)]++;

Also to consider: move the frequency analysis to Base/Utils/StringUtils.