DN-MC6000MK2: Sampler modesSampler modes + crossfader configuration

- Support 2 different sampler modes:
  - Hold (the new default)
  - Trigger
  The default sampler mode can be changed by setting DenonMC6000MK2.DEFAULT_SAMPLER_MODE appropriately.
- Assignment of crossfader to channels
- Modify contour of crossfader curve
- Delete unused booth level control