Echo Effect updates.

Add ramping to the beginning and end of delay buffer to prevent clicks.
Add Send parameter to control how much of dry is fed into the buffer.
Clamp echo buffer
Apply dry signal to wet portion to mimic send-effect behavior.
Change ordering of echo knobs.
Tweak dry/wet mixing to prevent overdriving of signal.
Merge pull request #269 from ywwg/echo-updates

Echo Effect updates.
DN-MC6000MK2: Initial revision
DN-MC6000MK2: Map some controls and the VU meter LEDs directly
DN-MC6000MK2: Looping

- Loops: Manual, beat and beatroll
- Censor: Use reverseroll instead of reverse
- Scratching: Don't modify keylock mode
DN-MC6000MK2: Track loading

- Solo cue mix when loading a track (can also triggered manually
  by pressing Shift + Cue Mix)
- Slip mode fixes (still does not always work as expected)
DN-MC6000MK2: Samplers and various improvements

- 16 samplers (4 per deck)
- Hotcue fixes
- Sync mode switching improvements
- Use cue mix LEDs as track load indicator
- Delete obsolete filter controls
DN-MC6000MK2: Polishing

- Global shift state
- Slip mode fixes
- Looping improvements
- Cue button controls AutoDJ fade/skip
- Shift + play triggers stutter play
- Internal refactoring of hotcues
- Delete obsolete constants & triggers
- Minor changes
DN-MC6000MK2: Many MIDI mapping fixes

- Many MIDI mapping fixes, mainly for [Channel3/4]
- AutoDJ changes
  - Shift + Cue triggers skip_next
  - Shift + Play triggers fade_now
- Internal refactorings & cleanup
DN-MC6000MK2: Only 4+4 samplers (shared between decks)

- MIDI debugging revealed that the sampler controls
  buttons/LEDs are sending/receiving on MIDI Ch0 (left side)
  and MIDI Ch2 (right side) independent of the currently
  selected deck.
- Automatically load selected track into empty deck
  or sampler when pressing the cue/play or the sampler
  button respectively
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
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
DN-MC6000MK2: Basic effect chain control

- Effect chain selector (need to press shift as labeled)
- Efx3 button enables/disables the effect chain
- Efx3 knob for dry/wet control
- Efx2 knob for master parameter control ("metaknob")
- Minor cleanup