Improve adding more css classes to the editor body, add "rtl", "ie7", "ie8",...

Improve adding more css classes to the editor body, add "rtl", "ie7", "ie8", "ie9" so they can be targeted in editor-style.css, see #19437

git-svn-id: https://core.svn.wordpress.org/trunk@20054 1a063a9b-81f0-0310-95a4-ce76da25c4cd