[MusicXML import] add instrument-sound to MIDI mapping
Some MusicXML files do not contain midi-instrument definitions for the score-parts, but use instrument-sound and/or virtual-instrument definitions instead. This is done by Sibelius 7 (Direct export).
Currently, MuseScore defaults the instrument to piano for these parts.
E.g. see https://musescore.org/en/node/73901.
Easiest solution would be to define an instrument-sound to MIDI mapping, as suggested by Michael Good in http://forums.makemusic.com/viewtopic.php?f=12&t=2229.
Note that I consider this must have for both 2.1 and 3.0.