Update QThread example: Mandelbrot

http://doc-snapshot.qt-project.org/5.0/threads-mandelbrot.html

Part of the QThread documentation cleanup project (see QTBUG-16358).

Background
In Qt 4.4, a default implementation was given to QThread::run(). This enabled a safer way to use QThread, compared to the old way of subclassing it; the latter approach led to many broken programs and spawned many questions on forums, blogs, IRC and the mailing list. However, years later, the documentation still advocates the subclassing approach.

Aim
This project aims to update documentation, to highlight currently-accepted best practices for using QThread.