New tutorial : Sortable lists

The symfony book keeps growing, and the latest update will please the ones looking for more AJAX tutorials and more model examples.

The new sortable lists tutorial describes two ways to implement a sortable list - a list that the user can reorder easily. The classic method uses arrows to move an item up and down, and the AJAX method transforms each item into a draggable widget.

The implementation is not hard, but if you want to keep a good MVC separation, you need to put most of the code dealing with the item order in the model. That's why this tutorial is also a good way to get familiar with Propel and the Criteria object.