On the Menus screen, the links to move menu items behave like buttons: they
perform an action so they should be real buttons to be correctly reported to
assistive technologies. Since they're logically grouped controls, they should
also be wrapped in a `<fieldset>` element for better semantics and accessibility.
Props Cheffheid.
Fixes#35578.
Built from https://develop.svn.wordpress.org/trunk@38770
git-svn-id: http://core.svn.wordpress.org/trunk@38713 1a063a9b-81f0-0310-95a4-ce76da25c4cd