Accessibility: Allow post boxes on the Dashboard and Classic Editor pages to...

Accessibility: Allow post boxes on the Dashboard and Classic Editor pages to be reordered by using the keyboard.

So far, it has been possible to rearrange into a new order the post boxes (also known as "widgets" on the Dashboard and "meta boxes" on the Edit post page) only by using a pointing device, for example a mouse.

This change adds new controls and functionality to allow the boxes to be rearranged also with the keyboard. Additionally, audible messages are sent to the admin ARIA live region to notify screen reader users of the reorder action result.

Props joedolson, anevins, antpb, audrasjb, xkon, MarcoZ, karmatosed, afercia.
Fixes #39074.

Built from https://develop.svn.wordpress.org/trunk@48373


git-svn-id: https://core.svn.wordpress.org/trunk@48142 1a063a9b-81f0-0310-95a4-ce76da25c4cd