QmlDesigner: Fix flipable.qml example file

tests/manual/qml/testfiles/flipable.qml

 import Qt 4.7
 
-Rectangle {
+Flipable {
     width: 640
     height: 480
+    front: Text {
+        text: "front"
+    }
+    back: Text {
+        text: "back"
+    }
+
 }