[QML frontend] Using pixelDelta instead of angleDelta for scrolling events 

angleDelta is pretty unusable, at least on Mac OS X : when I scroll with my mouse a little bit, the code code view scrolls too fast
using pixelDelta makes the scrolling far more usable
I'm also running Arch Linux, and `pixelDelta` works for me. I suspect choice of window managers and mouse/touchpad devices will make a difference here. I wonder if there's a sensible cross platform way to handle this.