[MusicXML export] transposition change to "in C" not exported

Currently the MusicXML exporter never exports transposition in C (chromatic = 0). This is correct for an instrument in C, but not when changing from an instrument with a different transposition to one in C.

Issue is in ExportMusicXml::writeInstrumentDetails(), which does not take the previous instruments transposition into account.

See attached files, where the transpose element is incorrectly missing for the clarinet in C.