Fix popup for existing image in IE8.

When a page already has an image on it and you select this and click the image button to change some options, you get a javascript error on IE8 because classList is not defined.

This pull request fixes it by calling some uglier code when classList is not defined.

I made this a pull request instead of simply committing on master because I am not sure if IE8 is supported in this version of Products.TinyMCE. I see no page about browser compatibility.