Add support for base64-formatted images in the default popup window

[erikedmark]

Problem
When the default popup is used that renders attribute based content, images of base64-format are not displayed.
The result is that a url with the value "undefined" is used as the src-value of the image tag and furthermore displayed as a broken link in the browser.

Expected behaviour/solution
If the image attribute value is of base64-format the the image shuld be used directrly as the src-property for the img-tag.