Fix running SDDM under a Plasma 5.0 desktop.

When running SDDM under a full Plasma 5.0 desktop, the integration plugin tries
to access the arguments to figure out a configuration file name.  However, as
argc in SDDM::GreeterApp was passed as a temporary into a QGuiApplication, which
takes argc as a reference, the reference to argc is no longer valid and the
application segfaults.  Fix by having GreeterApp take argc as a reference.
Merge pull request #242 from MJDSys/KDEFIX

Fix running SDDM under a Plasma 5.0 desktop.
Similar to c4cfeb6bbf8d, avoid crashes due to bad references.

Make sure argc is always valid when QCoreApplication access it, by keeping
it as a reference to argc from main.
Merge pull request #243 from MJDSys/argc_fix

Similar to c4cfeb6bbf8d, avoid crashes due to bad references.