[UX] combine different kinds of views link fields

Recommended by @BWPanda in https://github.com/backdrop/backdrop-issues/issues/3419

As an extra feature request, and slightly different from Drupal 8, what about also merging all the link fields together? E.g. remove 'Content: Delete link' and 'Content: Edit link' in favour of a more powerful 'Content: Link'.

Here's how it could look:

(I couldn't screenshot the select list open, but it'd have 'View', 'Edit', & 'Delete' as options.)