Accessibility: Improve the differences between revisions.

Color alone is not sufficient to visually communicate differences.

- adds `plus` and `minus` icons to the changed lines
- adds visually hidden text: `Added`, `Deleted`, and `Unchanged` to clarify the differences to assistive technologies users

Props birgire, audrasjb, kjellr, adamsilverstein.
Fixes #43532.

Built from https://develop.svn.wordpress.org/trunk@45729


git-svn-id: http://core.svn.wordpress.org/trunk@45540 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.3-alpha-45728';
+$wp_version = '5.3-alpha-45729';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.