Qt data conversion

Replaced lengthy QString to number conversions via stdlib (i.e. strtod/atoi) with native QString conversion methods (i.e. toDouble/toInt).
NW noticed that problem in #310 and I changed it accordingly in (hopefully) all other files because the old version is extremely unreadable.

I've noticed more potential for improvement in a lot of the files changed here (because a lot of them are a few years old by now) but lets please concentrate on the actual conversion issue here.