Moog ladder filter

This is a digital approximation of the famous Moog Ladder filter. 
It is based on the work of Antti Houvilainen. 
Code fragments found at:  http://musicdsp.org/showArchiveComment.php?ArchiveID=196

It has a double steep 24dB/Okt roll-off compared to the "Filter effect"
The resonance can be cranked up to self resonance, which produce a prominent sweeping effect.  

IMHO this is a nice addition to MIxxx and a good counterpart to the subtle "Filter effect" pull request, which is included in this PR as well. 

Both are designed to be an EQ replacement if desired.


It is checked in ControlLogPotmeterBehavior::ControlLogPotmeterBehavior() 

```
if (minDB >= 0) {
    qWarning() << "ControlLogPotmeterBehavior::ControlLogPotmeterBehavior() minDB must be negative";
    m_minDB = -1;
}
```