Smoothly ramp changes in gain.

https://bugs.launchpad.net/mixxx/+bug/854082

Add ```copyWithRampingGain```, ```applyRampingGain``` and ```addWithRampingGain``` methods to ```SampleUtil``` and use them to ramp changes in gain in various places:

  * deck EQs
  * deck pregain 
  * channel volume
  * channel orientation
  * crossfader
  * master volume
  * headphone volume 
  * head/mix 
 
Still remaining to be done:
  * balance