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. 

A Null value is not possible. There must be a default to link the value to the super knob. Without this patch the neutralHint was 0.0. It is only required to change this (call setNeutralHint) when there is something special in linking.