Use orchestral sounds for orchestral percussion

Currently, a number of orchestral percussion instruments play back with equivalent standard drumset sounds by default. MuseScore's FluidR3Mono, like most GM SoundFonts, comes with an "Orchestra Kit", but this is currently not used. This issue was raised at https://musescore.org/en/node/96436.

It should be noted that S. Chris Collins took this into account when designing GeneralUser, and put several orchestral percussion sounds into the standard drumset.