fix high dpi scaling with qt5.6

use the new Qt::AA_EnableHighDpiScaling introduced in Qt5.6

Change-Id: I258047bcc54413ce344103e4e90d5dc7e3756369
Tuleap: #916

main.cpp

@@ -62,7 +62,7 @@ Console()
 int
 main(int argc, char *argv[])
 {
-    qputenv("QT_AUTO_SCREEN_SCALE_FACTOR", "1");
+    QGuiApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
 
     QApplication a(argc, argv);