"Export Parts" should convert the filename as per the same rules as the new score wizard

I.e. change spaces (and tabs?) to "_", umlauts and other special characters into their ASCII transcriptions (e.g. "ö" → "oe")

Otherwise a part named e.g. "Flöte" (Flute) would result in non-portable filenames (e.g. Scorename-Flöte.pdf")

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