Improve layout widget references in parallax demo

Use the selector API to prevent having to set layout data after the
referenced widget has been created, which was not intuitive.

Change-Id: Ifb39c405e842667c37eda97154f1bfc90dc03486

examples/parallax/parallax.js

@@ -15,6 +15,7 @@ var imageTextView = tabris.create("ImageView", {
 }).appendTo(scrollComposite);
 
 var contentComposite = tabris.create("Composite", {
+  layoutData: {left: 0, right: 0, top: ["#titleComposite", 0], height: 1000},
   background: "white"
 }).appendTo(scrollComposite);
 
@@ -39,11 +40,10 @@ tabris.create("TextView", {
 }).appendTo(contentComposite);
 
 var titleComposite = tabris.create("Composite", {
+  id: "titleComposite",
   background: "rgba(255,152,0,0.9)"
 }).appendTo(scrollComposite);
 
-contentComposite.set("layoutData", {left: 0, right: 0, top: [titleComposite, 0], height: 1000});
-
 tabris.create("TextView", {
   markupEnabled: true,
   text: "<b>The perfect side dish</b>",