Revert one change as it is only available after Qt5.14

Core/PluginLoader.cc

@@ -317,7 +317,8 @@ void Core::loadPlugins()
    */
   {
       QStringList list = QString::fromUtf8(cmake::static_plugins).split("\n");
-      QSet<QString> staticPlugins = QSet<QString>(list.begin(),list.end());
+      // This warning couöld be fixed when the minimal Qt Version is >= 5.14
+      QSet<QString> staticPlugins = QSet<QString>::fromList(list);
       for (int i = 0; i < pluginlist.size(); ) {
           const QString bn = QFileInfo(pluginlist[i]).fileName();
           if (staticPlugins.contains(bn)) {