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.
We cannot know is this is true after later changes. We may ask the EffectChainManager via a new function 
m_pEffectsManager->getEQEffectRack()