MSCX format: rename <instrumentID> to <MusicXMLid>
MSCX files contain syntax like the following:
<Instrument id="flute"> <longName>Flute</longName> ... <instrumentId>wind.flutes.flute</instrumentId> ... </Instrument>
id="flute" attribute is MuseScore's internal ID for the flute instrument, whereas
<instrumentId>wind.flutes.flute</instrumentId> is actually the MusicXML ID. It should be renamed
<MusicXMLid> to avoid confusion here and in the code.
Alternatively, the MusicXML could simply be removed from the MSCX format because we now have the MuseScore ID available and can use that to get the MusicXML ID from instruments.xml.