Fix TinyMCE Views in IE8 and 7:

- Add fallback background images for dashicons.
- Fix hiding the clipboard while still allowing the contents to be selected.
- Work around lack of border-box in gallery styles.
(Includes precommit cleanup for theme.js)
Props gcorne, fixes 27546
Built from https://develop.svn.wordpress.org/trunk@28062


git-svn-id: https://core.svn.wordpress.org/trunk@27894 1a063a9b-81f0-0310-95a4-ce76da25c4cd