Add specific CSS class to glossary entry links

This issue is about adding a specific CSS class to the rendered items, such as glossary-entry, so that the links pointing at glossary entries can be targeted by a dedicated selector both for applying a specific design or for implementing a specific behaviour (such as popups as in GLOSSARY-3). Currently only the wikigeneratedlinkcontent CSS class is present so there's no way to target specifically glossary links.

NB: the CSS class should be added also when using the glossaryReference macro.