list.plugin.Animate: remove the need to specify DOM ids for each child node

This is especially needed for future component based list demos which are using this plugin.

The vdom engine relies on DOM ids to check for move OPs. When executing triggerTransitionCallback(), we regenerate all items to remove the ones which got faded out and restore the item indexes.

To resolve this, createItem() needs to check for existing items within the virtual DOM and use e.g. util.VDom: syncVdomIds() to restore existing child ids for us.