Super softtakeover

This introduces soft takeover for the super knob effect parameter connection. 
The super knob jumps to the current value of the linked parameter on effect load. 
https://bugs.launchpad.net/mixxx/+bug/1329063
https://bugs.launchpad.net/mixxx/+bug/1335355

It also includes some soft takeover refactoring 
This does not matter, because we use the result only as internal value for compare against the threshold for soft-takover. The current implementation keeps the current state in master. But I will have a look how much work it is to fix this. At least a comment will would be nice.