Link fix

This PR is another step towards combined  effects. 
It replaces the invert linked hint and introduces a neutral hint instead. This set a point on the knob parameter that are adopted at neural point on the super knob. 
The super knob can be driven in two modes, a neutral center mode, usually used for EQs and Filter effect, or a neutral left mode used for effects like bit-crusher. 
If you like to combine a bit-crusher with an EQ, you can set the link type of the bit-crusher knobs to new left, right or left/right mode where the neutral point (1.0) of the bit-crusher is mapped to 0.5 at the super knob.  
All this can tested with the Shade skin. 

That is not possible. 
And that thats reason why I am working on a new Filter effect. 
https://github.com/daschuer/mixxx/tree/filter-effect. 

We head requests to allow something like Xone DB4 
https://bugs.launchpad.net/mixxx/+bug/1052019

It will have a dedicated LPF knob a HPF knob and a Q Knob. 
One knob is linked on the left half tho other on the right half. 

With his model you are able to combine this with other effects in a sensible way. 
You may want to combine the filter and the bit crusher ... 
Such things are not possible in the master version yet.