Accessibility: Improve the focus style for Windows High Contrast mode in...

Accessibility: Improve the focus style for Windows High Contrast mode in various parts of the admin interface.

Continues the introduction in core of new focus styles dedicated to Windows High Contrast mode. The new styles use a transparent CSS outline.
This change covers some parts of the interface for the meta boxes, Widgets, and the Customizer.

Props joedolson, kjellr, antpb, mikeschroder, Hareesh Pillai.
See #41286, #45910.
Fixes #47117.

Built from https://develop.svn.wordpress.org/trunk@48293


git-svn-id: https://core.svn.wordpress.org/trunk@48062 1a063a9b-81f0-0310-95a4-ce76da25c4cd