Customize: Fix visible edit shortcuts for `wp_nav_menu()` instances using the...

Customize: Fix visible edit shortcuts for `wp_nav_menu()` instances using the `menu` arg (such as in the Custom Menu widget) instead of the `theme_location` arg.

Also fix logic for `focus-control-for-setting` handler to focus on the first control (lowest `priority` value) associated with a given setting instead of the last control encountered when iterating over all controls, as this ensures the first control in a `nav_menu` section is focused rather than the last one.

Props westonruter, sirbrillig.
See #27403.
Fixes #39101.

Built from https://develop.svn.wordpress.org/trunk@39622


git-svn-id: https://core.svn.wordpress.org/trunk@39562 1a063a9b-81f0-0310-95a4-ce76da25c4cd