don't change effect when scrolling over WEffectSelector

I see the scrolling feature of WComboBox is supposed to improve accessibility but here it can be realy annoying: a few times I accidentially scrolled with my touchpad when hovering an effect seletor. When this effect is currently active this can be a show killer, say you switch from a centered filter effect to BitCrusher..
Also any custom Super knob link mapping as well as custom parameters are reset.

The entire effect selector area is clickable thus I think it's sufficient to open the drop-down on click for switching effects and we should disable the scroll feature.