Add DUI command to change an image

Eran Yaniv pointed on Diaspora that there's no DUI command to modify the source of an image after it has been added to the canvas. This can be done by a command like the following, but it makes sense that there's a command that doesn't require non-exported DUI commands:

`dui item config DYE beans_img -image [dui::image::load photo "DYE-beans_img-new" [dui::image::find "bean_Insight.png"]]`