Disable session management properly

The attribute needs to be set before the QGuiApplication is created or
else it doesn't have any effect.

BUG: 415874

discover/main.cpp

@@ -79,11 +79,13 @@ void processArgs(QCommandLineParser* parser, DiscoverObject* mainWindow)
 
 int main(int argc, char** argv)
 {
+    // needs to be set before we create the QGuiApplication
+    QCoreApplication::setAttribute(Qt::AA_DisableSessionManager, true);
+
     QApplication app(argc, argv);
     app.setWindowIcon(QIcon::fromTheme(QStringLiteral("plasmadiscover")));
     app.setAttribute(Qt::AA_DontCreateNativeWidgetSiblings);
     app.setAttribute(Qt::AA_UseHighDpiPixmaps, true);
-    app.setAttribute(Qt::AA_DisableSessionManager, true);
     KCrash::initialize();
     KQuickAddons::QtQuickSettings::init();
     KLocalizedString::setApplicationDomain("plasma-discover");