[skip ci] improve possibility to create a QApplication in a thread

src/Main/FreeCADGuiPy.cpp

@@ -109,6 +109,9 @@ FreeCADGui_showMainWindow(PyObject * /*self*/, PyObject *args)
 #if (QT_VERSION >= QT_VERSION_CHECK(5, 9, 0))
                 QApplication::setAttribute(Qt::AA_ShareOpenGLContexts);
 #endif
+                // This only works well if the QApplication is the very first created instance
+                // of a QObject. Otherwise the application lives in a different thread than the
+                // main thread which will cause hazardous behaviour.
                 QtApplication app(argc, argv);
                 if (setupMainWindow()) {
                     app.exec();