highui: qt - enable hidpi by default

can be overriden by environment:
QT_AUTO_SCREEN_SCALE_FACTOR=0

modules/highgui/src/window_QT.cpp

@@ -529,6 +529,9 @@ static int icvInitSystem(int* c, char** v)
     //"For any GUI application using Qt, there is precisely one QApplication object"
     if (!QApplication::instance())
     {
+#if QT_VERSION >= QT_VERSION_CHECK(5, 6, 0)
+        QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling, true);
+#endif
         new QApplication(*c, v);
         setlocale(LC_NUMERIC,"C");