Port the UI functionality from Entity API

One feature provided by Entity API in Drupal 7 is the ability to generate admin pages and menu items for managing your custom entity, including a page under content that lists all of the entities and links to their edit forms.

This does seem like a useful feature that we should implement. However, providing this UI should probably be handled in its own separate module, either in contrib or as a submodule of Entity in core.

If we do include Entity UI functionality in core, perhaps we can look at what it would take to use it to generate the admin interface for all core entities.