Accessibility: Add focus styles for Windows High Contrast mode.

Introduces in core new focus styles dedicated to Windows High Contrast mode. The 
new styles use a transparent CSS outline as already explored in the Block Editor.
This first part covers some of the main user interface controls like buttons, links,
and media views elements. Other parts will be addressed in the next future.

Fixes #41286.

Built from https://develop.svn.wordpress.org/trunk@44544


git-svn-id: http://core.svn.wordpress.org/trunk@44375 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/css/admin-bar.css

@@ -53,7 +53,6 @@ html:lang(he-il) .rtl #wpadminbar *  {
 #wpadminbar a:hover,
 #wpadminbar a img,
 #wpadminbar a img:hover {
-	outline: none;
 	border: none;
 	text-decoration: none;
 	background: none;
@@ -71,7 +70,11 @@ html:lang(he-il) .rtl #wpadminbar *  {
 #wpadminbar textarea,
 #wpadminbar div {
 	box-shadow: none;
-	outline: none;
+}
+
+#wpadminbar a:focus {
+	/* Inherits transparent outline only visible in Windows High Contrast mode */
+	outline-offset: -1px;
 }
 
 #wpadminbar {