Fix and simplify parallax demo
Fix and simplify parallax demo

- Remove the fixed height of header bar
- Reorder widgets to match their visual order
- Use scaleMode instead of setting explicit image bounds,
  remove hardcoded image scale factor
- Use single layout properties (left, top, etc.) instead of
  layoutData, only update the variant parts

Fix #503

Change-Id: I7d56481cee9e1357543b584fff95baf51782e655

eclipsesource/tabris-js,eclipsesource/tabris-js,eclipsesource/tabris-js