QRM: allocate UI on the heap

MainWindow has ModelFactory as a member, and that again has large arrays
as members. The stack on Windows cannot handle that, and the test fails
to start.

Allocate the UI on the heap.

Pick-to: 6.10 6.10.0
Change-Id: I8659fd7bc267f133e66e5620a430ba32b7abae2a
Reviewed-by: Maycon Stamboroski <maycon.stamboroski@qt.io>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

tests/manual/corelib/itemmodels/qrangemodel/main.cpp

@@ -605,8 +605,8 @@ int main(int argc, char *argv[])
 {
     QApplication app(argc, argv);
 
-    MainWindow mainWindow;
-    mainWindow.show();
+    auto mainWindow = std::make_unique<MainWindow>();
+    mainWindow->show();
 
     return app.exec();
 }