[imp] Fields layouts: convert JFormFieldList

Issue tracker:
http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=32229

This opens the way to follow to start using layouts to render fields. This would allow override fields in templates i.e.

You can customise the layouts specifically also for a field instance with the `layout` attribute. Example:

```xml
<field
	name="caching"
	type="list"
	default="2"
	label="COM_CONFIG_FIELD_CACHE_LABEL"
	description="COM_CONFIG_FIELD_CACHE_DESC"
	required="true"
	filter="integer"
	layout="field.mylayout"
	>
		<option value="0">COM_CONFIG_FIELD_VALUE_CACHE_OFF</option>
		<option value="1">COM_CONFIG_FIELD_VALUE_CACHE_CONSERVATIVE</option>
		<option value="2">COM_CONFIG_FIELD_VALUE_CACHE_PROGRESSIVE</option>
</field>
```

That will render a `list` field but instead of using the standard `joomla.form.field.list` it would use the layout `field.mylayout`. This is useful for example if we want to add some AJAX logic. Instead of creating our own field we just create a custom layout.

Also for the recent chosen issues this would allow us to add a class to every single select field (something not recommendable :D )