Rethink CP nav customisation to improve UX and reduce confusion.

[robdekort]

When you group items using the CP nav feature you can't set icons, which is an open issue (#9667). However something else that's a bit confusing is that collections and taxonomies you move out of a certain main nav item remain in the overview table.

In this case I made a group that contains two taxonomies and one collection:

But in the collection and taxonomy overview tables you still see them listed, when they're removed from the sidebar:

This is a bit inconsistent and possibly confusing. It might be better for them to be removed from the overview table as well.