Accessibility: Menus: Make tab panels in `Add menu items` section more...

Accessibility: Menus: Make tab panels in `Add menu items` section more accessible for keyboard users.

Props audrasjb, afercia.
Fixes #49211.
Built from https://develop.svn.wordpress.org/trunk@47209


git-svn-id: https://core.svn.wordpress.org/trunk@47009 1a063a9b-81f0-0310-95a4-ce76da25c4cd