Add 10mm rectangle to screen example to verify Screen.pixelDensity

Change-Id: Icaca370be3eae3ed50971e5bd2f6f5f7278c2962
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

examples/quick/window/CurrentScreen.qml

@@ -101,5 +101,12 @@ Item {
         Shared.Label { text: "primary orientation" }
         Shared.Label { text: orientationToString(Screen.primaryOrientation) + " (" + Screen.primaryOrientation + ")" }
         //! [screen]
+
+        Shared.Label { text: "10mm rectangle" }
+        Rectangle {
+            color: "red"
+            width: Screen.pixelDensity * 10
+            height: width
+        }
     }
 }