Improved the way we determine/retrieve the plugins directory [ci skip]. As a result, we can once again run OpenCOR from Qt Creator (from Linux at least since we are still having a problem on Windows with the Qt platform plugin "windows" not being found). On OS X, it hadn't stopped working since it's a bundle. Otherwise, the reason it stopped working is because of our use of QCoreApplication::setLibraryPaths().
Merge branch 'master' of github.com:opencor/opencor