Use qt.conf to override plugin path on Mac OS

Summary:
QCoreApplication::addLibraryPath is called before QCoreApplication
was constructed and this caused the returned string to be different
depending from which working directory it was called and not always
the intended binary path.
Using qt.conf has a fixed prefix relative to the binary path
inside the application bundle and therefore is easier to be used
with a relative path.

Reviewers: kbasan, msutcliffe

Reviewed By: msutcliffe

Differential Revision: https://dev.swift-project.org/D18

samples/blackmiscdbus/main.cpp

@@ -38,9 +38,6 @@ int main(int argc, char *argv[])
     // is just for testing, I did not split it up
 
     BlackMisc::registerMetadata();
-#ifdef Q_OS_MAC
-    QCoreApplication::addLibraryPath(CDirectoryUtils::applicationDirectoryPath());
-#endif
     QCoreApplication a(argc, argv);
     QTextStream out(stdout, QIODevice::WriteOnly);
     QTextStream qtin(stdin);