Excellent point. For a short moment you made me feel incredibly stupid.
I did a little research just now and saw that the "toDouble()"-method was introduced in Qt4.6.
The interface was written when Qt4.5 was just out (because they added the XmlValidator in that version), so the simple answer is, that the method didn't exist back then.

I will change that anyway because it'll make things much more readable. Thanks for the tip.