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.
Ok, then it seam there is an underlying issue. Probably something to do with the memory leak you are facing in the open bug. Please leaf a comment here and we can issue that in a separate commit.