[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 running Archlinux, and when I changed `angleDelta` to `pixelDelta` I wasn't able to scroll anymore.

I tried `angleDelta` with a smaller multiplier, that's better but the transition is still too long.