Add arbitrary class information to the theme layout elements (page, section and cell).

In order to make it easier to incrementally contribute small stylesheet overrides by addons and customization packages from Studio it is necessary to make the layout of the default Nuxeo themes (Admin Center, DM, DAM, CMF...) carry semantically named class names on layout nodes.

To do so we want to make it possible for the layout designer to pass arbitrary class names that are use as CSS class on the matching HTML elements generated by the theme engine without any kind of prefixing or namespace scoping. It's the responsibility of the theme designer to come up with good CSS class names.