Graphic EQ Effect

Thiss PR adds a 10 band static equalizer based on Biquad filters. The implementation of the Biquad filter is based on @daschuer's iireqs branch.
Thanks for noticing that. It was a mistake to use append. ```setFrequencyCorners``` should be called for each existing filter. Q or center frequencies cannot change during Mixxx's lifetime so we can call ```pauseFilter()``` for each filter inside this method.