Utility class representing links to different views of a record for CMS authors, usually for {@link SiteTree} objects with "stage" and "live" links.

Useful both in the CMS and alongside the page template (for logged in authors).
The class can be used for any subclass implementing the interface.
New item types can be defined by extending the class,
for example the "cmsworkflow" module defines a new "future state" item with a date selector
to view embargoed data at a future point in time. So the item doesn't always have to be a simple link.