Static 8 Band Master EQ 

This PR adds a static 8 Band EQ to the EQ Preferences. 
It can be later extended to a configurable Effect Rack, that is intended to setup before the gig, during sound check.  
For Mixxx 1.12 this should already cover the most common use-case: to equalize the sound of the sound-system. 

It is on top of the filter_effect PR