resizeWindow doesn't work properly with Qt backend (Bug #4244)

I noticed that the function resizeWindow doesn't work properly when using the Qt backend (Either with Qt4 or Qt5), while it works as expected with the GTK backend.

A simple code snippet like this can reproduce the situation:

#include <opencv2/highgui/highgui.hpp>

int main()
{
        cv::Mat img(400, 400, CV_8UC1, cv::Scalar::all(255));

        cv::namedWindow("img", cv::WINDOW_NORMAL | CV_WINDOW_KEEPRATIO);
        cv::moveWindow("img", 0, 0);
        cv::resizeWindow("img", 800, 800);

        cv::imshow("img", img);
        cv::waitKey();
}

I attached to screenshots depicting the my desktop just after running the application and having opencv compiled with the different backends.