Release the HelpWindow QProcess while MantidPlot is shutting down.

If left until the HelpWindow::destructor then the MantidPlot process hangs on
shutdown inside the QProcess destructor waiting on a mutex. It is most
likely that this because the Qt framework is now too far gone at the
point when the singletons are released. Refs #6873
Merge remote branch 'origin/feature/6873_hanging_helpwindow_singleton'