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? 
why the change from a qvariant to a double?