ui: support HDPI screen

- enable Qt device pixel ratio detection, with a correctly written code
the scale is painless (Use of QFontMetrics, ...)

Change-Id: I868f8437c93a64651cf7ad8b0aa65804c6ad5295
Tuleap: #143

main.cpp

@@ -57,6 +57,8 @@ Console()
 int
 main(int argc, char *argv[])
 {
+    qputenv("QT_DEVICE_PIXEL_RATIO", "auto");
+
     QApplication a(argc, argv);
 
     auto startMinimized = false;