LPS-35472 Add classNameIds to PortletDataHandlerControls

Hey Julio,

This PR includes:

* An extension to PortletDataHandlerControl so that for every control a help-message can be easily defined and obtained. I've used it for the "structures" control in Web Content, and we can extend it to any other control that requires extra info.

* Adding the classNameId to every control defined in the PortletDataHandlers, whenever it's possible. With this change, those controls will display the number of available items in the export form, or will be hidden if there's none.

Thanks