[imp] Fields layouts: convert JFormFieldList

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 )