Separate the menu settings from the 'add link' button

Problem/Motivation

During usability research sessions, some participants thought that the menu settings 'title' field was actually the new menu link title field. They filled out the title and then pressed the 'Add link' button, expecting a menu link to be created.

This problem originated with #663946: Merge "List links" page into "Edit menu" page → where the "Edit menu" tab which had just menu title and description, was merged with the "Manage links" tab which had the links that users thought of as being the real substance of the menu. Bringing these together brought the UI into line with user's expectations and mental model.

However, it introduced a problem of conflicting UI elements. The "Add link" button goes at the top by Drupal convention, and relates to the links list. The menu "title" & "description" does not relate directly to the links list, but uses the same "Save" button as the "enabled" checkboxes that are part of the links list. The "Add button" was far closer to the menu "title" & "description" than the "Save" button, although it is has no relation to them.

The result was that it is easy for users to think that the title was for the new link to be added, or possibly also to click the add button unthinkingly when trying to save the menu title.

Proposed resolution

Place the Menu title and description into a collapsible fieldset title "Menu title" and move that to below the listing and above the save button.

Two other minor details have also been folded into this issue:
1) The menu title has been labelled "Menu title" not merely "title".
2) The action "Add link" in various places has been updated to the name used on its destination page, "Add menu link".

Remaining tasks

Postponed on 📌 [policy] Determine design for forms that have multiple actions Active
Code review
Product manager sign off

User interface changes

API changes

None

Data model changes

None

Wider issues

There is no good Drupal design pattern for forms placed on listings, so resolution of this issue may set a pattern to be used elsewhere.

This issue may also hint at a broader problem with the styling of action links, they look similar to general styling of form submission buttons.

Two other places in the Drupal UI have been identified which have the same basic UX issue, of an entity that has only a title, description, and children. These should probably have their UI updated to match the pattern established in this issue.
📌 Make shortcut add and edit form titles better Postponed
📌 Merge "list terms" page into "edit vocabulary" page Postponed