Add icon to the help button in distraction-free writing.

props ocean90.
fixes #26450.

Built from https://develop.svn.wordpress.org/trunk@26722


git-svn-id: http://core.svn.wordpress.org/trunk@26611 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/css/editor-rtl.css

@@ -743,6 +743,7 @@ span.mce_redo:before {
 	content: '\f172';
 }
 
+span.mce_help:before,
 span.mce_wp_help:before {
 	content: '\f223';
 }