Use Qt 5's QStringList::join(QChar)

Less typing and less cycles than join(QString) where appropriate

Change-Id: I6ebc0e17e4d7fd9845864dd95b7de4ba4dad6906
Reviewed-by: Eike Ziller <eike.ziller@digia.com>

src/libs/extensionsystem/pluginmanager.cpp

@@ -517,7 +517,7 @@ QString PluginManager::serializedArguments()
             rc += QLatin1Char(':');
             rc += ps->name();
             rc += separator;
-            rc +=  ps->arguments().join(QString(separator));
+            rc +=  ps->arguments().join(separator);
         }
     }
     if (!d->arguments.isEmpty()) {