Accessibility: Menus: Properly disable form controls on the Menus screen that...

Accessibility: Menus: Properly disable form controls on the Menus screen that cannot be used until at least one menu is created.

This ensures that the actual UI controls state matches the visually conveyed state.

Props afercia, audrasjb, dinhtungdu, sathyapulse, simonjanin, welcher, SergeyBiryukov.
Fixes #48409.
Built from https://develop.svn.wordpress.org/trunk@46842


git-svn-id: https://core.svn.wordpress.org/trunk@46642 1a063a9b-81f0-0310-95a4-ce76da25c4cd