Ability to add menu links directly to the Site Settings Group list

Updated a bit as I see ECK does a mix of Groups (Bundles) and directly to the entity. Would suggest we stick to just Groups here so the link from automated menus has a consistent behaviour to avoid any UX issues there. Then unless there is a good use case for individual single items, a site builder can manually add a few more specific ones if desired.

Okay tests passing. Can always do further work in follow-ups if more features desired.

Automatically closed - issue fixed for 2 weeks with no activity.