Dedicated filter knob (Color Fx) / four Band EQ

This PR is useful for all controllers with a dedicated Filter knob or a fourth EQ Band. 
It is on top of the EQ Rack branch https://github.com/mixxxdj/mixxx/pull/364, which should be merged first. 
A second effect slot is introduced for each deck, which can be used for filter, bitcrusher or other fading effects. This is almost similar to the Pioneer DJM-850 Color effects.
This new filter slot can only controlled by the reintroduced filterDepth = SuperButton. 
If we later add the highly requested four band Mixing EQ, it will have is sub band bound to the super knob to work nice with this concept.