Per menu attributes

Problem/Motivation

Most Drupal websites use more than one menu and some attributes might be only useful in some of them.

Proposed resolution

Add an optional key to the attributes to only show them for some menus. As menu link items can be moved from one menu to another using the "parent link" field, this might need to rely on some JS to make fields show/hide when the parent field is changed. Luckily, values of this field are prefixed by the menu name so it should not be that hard.

Remaining tasks

Discuss
Implement
Enjoy