DN-MC6000MK2: Filters and initial effects support

- Effect units (ordered according to physical controller layout
  from left to right):
   - EffectUnit1: Left FX1
   - EffectUnit2: Left Filter
   - EffectUnit3: Right Filter
  - EffectUnit4: Right FX2
- Filters are fully functional
- FX1 and FX2 can be assigned to decks
- Minor changes of VuMeter sensitivity