regression: qsortfilterproxymodel::reset doesn't invalidate the model (Windows)

This problem is introduced by merge request 416.
Because the connection between modelReset signal and invalidate slot was removed, calling model->reset() doesn't invalidate the model anymore.
You can see from the test case that when you click on the directory in the left tree view, the right listview should show the files inside the directory. But it doesn't show files correctly when you build it against 4.7.
It works against 4.6.x.