Separate MIDI generation into score vs. accompaniment

[infojunkie]

As per #54 , we're moving away from trying to generate an MMA file that includes both the score music and accompaniment. Verovio does a great job at generating the score MIDI, and is actively maintained, so the idea is to offload the score generation to Verovio, and focus on accompaniment generation only with MMA.

We'll thus end up with 2 MIDI files that need to remain mutually consistent and get recombined. The biggest challenge for consistency is probably ensuring that the unrolling / expansion algorithms in Verovio and unroll.xsl function equivalently.

There are several options for recombination:

• Use MMA's own MIDIInc directive if it is robust enough
• Merge the files into one using external tools such as midisox, or equivalent home-made tool
• Send 2 files to the player that can be played back concurrently with some frontend modifications