MelodyPart perf improvement

[oggy22]

MelodyPartList can improve perf by refactoring as: insert

class MelodyPartList
{
   struct NotePartChunck
   {
      NoteWithDuration nwd;
      MelodyPart mp;
   }
   private List<NotePartChunk> npc;

  void AssertValid()
  {
    for (int i=0; i<npc.lenght; i++)
    {
        if (i!=0) Debug.Assert(npc.nwd.Count != 0);
        if (i!=npc.Lenght-1) Debug.Assert(npc.mp.Count != 0);
    }
  }
}