LPS-33626 portlet-data.xml related bugfix and refactoring

Hey Julio,

This PR is fixing a bug in the staging environment (described in the ticket with root case LPS), and it also adds some refactoring to the way of handling the portlet-data.xml files.

Now it's much clearer and more generic, we leave to the framework to deal with some parts of the xml operations, and also the root element for that xml is now accessible from everywhere in the portlet and staged model data handlers.

I've first applied the changes in different commits, then applied the new pattern to one portlet data handler as an example and then to the rest.

I've made an exception with the page ratings because it originally saved only one long value in the xml which is far from the well formed xml :)

If you have any questions just let me know.

Thanks,

Máté