[Development] Backporting the "stop unloading plugins" change to 5.6

[Thiago Macieira]

Hello

We've got a number of issues that got fixed in 5.7 by the change that made 
QFactoryLoader stop unloading plugins (notably, the Network Manager bearer 
plugin). Given that QtDBus in 5.6 is now heavily threaded, a number of new 
issues have cropped up. I've managed to fix some, but reports from Ubuntu 
developers indicate the latest fix only uncovered the next issue.

If we do stop unloading, the problem goes away.

So, ok to backport?

See https://codereview.qt-project.org/172014 (original https://codereview.qt-project.org/140750)
-- 
Thiago Macieira - thiago.macieira (AT) intel.com
  Software Architect - Intel Open Source Technology Center