Add DUI command to change an image

[ebengoechea]

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"]]