[PluginLoader] Use operator<< for finalArgs instead of initializer list

When appending a list to an existing list using append or operator<< each item in the list
is added individually whereas with initializer list it just adds the entire list as one entry.

This caused the applet args to never be empty usually and resulted in externalData being
emitted in init() with a QVariantList as argument.

While this didn't do much for most applets it did break FolderView (and potentially others)
which updates its URL in response to this signal and stored the "data" it got in
plasmoid.configuration.url. FolderView's model did not accept this broken data [1] so the
folder would still work initially. On next startup, however, it loaded said value from
config and just went blank.

[1] "file:///usr/share/plasma/plasmoids/org.kde.desktopcontainment/contents/ui/FolderViewLayer.qml:202:14: Unable to assign QJSValue to QString"

REVIEW: 129158

src/plasma/pluginloader.cpp

@@ -234,7 +234,8 @@ Applet *PluginLoader::loadApplet(const QString &name, uint appletId, const QVari
     if (!applet) {
         //qCDebug(LOG_PLASMA) << name << "not a C++ applet: Falling back to an empty one";
 
-        QVariantList allArgs = { p.metadata().fileName(), appletId, args };
+        QVariantList allArgs;
+        allArgs << p.metadata().fileName() << appletId << args;
 
         if (p.metadata().serviceTypes().contains(QStringLiteral("Plasma/Containment"))) {
             return new Containment(0, allArgs);