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 may need m_isDeckEQ as well to exclude the graphic EQ or other EQs with more than three knobs.