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. 

I think this should be a QVariant -- otherwise we can't represent the null (no hint known) state that most external plugins will have.