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

The way you've done it (talking to the Effect's parameter directly) is the right way for this code I think.

For the LV2 branch this doesn't make sense. You're not tweaking an instantiated effect in that case -- you're tweaking the manifest that is used to produce instantiated effects.