allow mashup of two songs

[monking]

There are two players, to make skipping seamless...so use them to queue up two songs to play from one composition, sampling sections from both. This would require a different structure of playlist. For example:

"tracks":[
  {"id":"ttng_minute_snake"},
  {"id":"tortoise_magnet_pulls_through"}
]

"tracks":[
  {
    "composition":[
      {
        "id":"ttng_minute_snake",
        "ab":[0,5]
      },
      {
        "id":"tortoise_magnet_pulls_through",
        "ab":[15,21],
        "x": 4
      },
      {
        "id":"ttng_minute_snake",
        "ab":[92,110],
        "x": 2
      }
    ]
  }
]