EQ Rack

For more details, check: http://www.mixxx.org/wiki/doku.php/eq_rack

This PR still suffers from https://bugs.launchpad.net/mixxx/+bug/1335823. To reproduce it, check "Show All Effects" checkbox several times.
Ah, now I see. Can you try to reduce code duplication. 
E.G introduce a common addEffectChainSlot(int numEffectSlots) and or similar.