Thread priority don't work on Symbian (Mandelbrot example is busted)

If you call the start(LowPriority) method on a QThread derived class, the Symbian port silently discards the specified priority.

See the code for QThread::start(Priority priority) in
src\corelib\thread\qthread_unix.cpp

Thread priority support is needed to make any non-trivial app work with any degree of UI responsiveness on Symbian.