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. 

Edit:
A testing skin can be found here: https://github.com/mixxxdj/developer_skins/commit/6f223e2f030776fff294e544d944a012e174fa1a

Minor issue -- adding a 2nd standard effect rack will replace the first since they will both have an empty group here.