Show selected state for video iframe elements (#6564)

http://www.tinymce.com/develop/bugtracker_view.php?id=6564
Added iframes get edited/added with the media plugin but aren't getting
added to the selector which triggers the highlight for the menu button.
Added in the iframe selector so that when you click on an added
video/iframe, the media button highlights.
Merge pull request #323 from monkeydiane/addVideoHighlight

Show selected state for video iframe elements (#6564)