Form fields update

* button: added `title` attribute (used for tooltip)
* groupedbuttom: we need to `htmlentities` or it will crash the XML parsing (if we add a link for example)
* list & model: refactorized `parseFieldTags`, allow to add new tag w/o rewritting `getRepeatable`
* ordering: added `icon` and `class` attribute, allow to use on non-bootstrap 2.3 templates
* checkboxes: new fields based on Joomla `checkboxes`

If everything ok, I'll update the doc.