replace terminated signal with finished

Remove the QThread::terminated() signal
The signal is removed from the API; all references to it are removed
from documentation; the unit test that checks for its emission is
modified to listen for QThread::finished() instead.

The QThreadPrivate::terminated flag is also removed, as it served no
purpose other than to trigger the emission of QThread::terminated()

As discussed at
http://lists.qt-project.org/pipermail/development/2012-October/007216.html
the signal is not guaranteed to be emitted after every termination,
rendering it useless.