Kill buttons2

This adds kill buttons to all three new three band EQs. 
This commit is the successor of https://github.com/mixxxdj/mixxx/pull/297 and based on it. 

I know there is a pending discussion if this is the right way to support push buttons.
So this is only a base to implement the final solution for 1.12

Any hints what to improve to be mergeable? 
We can only control Effect Parameters by Control Objects which are double 
All expected values like int and bool can be expressed lossless by a double. 
LV2 and VST is using float. I do not expect a later requirement for a QVariant.
So there is no need for the overhead QVariant introduces.