Show 'secondary' images on a canvas

[ahankinson]

A canvas should always have at least one image associated with it, but sometimes it may have more than one (for example, if a photo has been taken with UV or infrared light). These images may or may not be the same size as the canvas.

It would be good if we could view multiple images on a canvas. This could be implemented as a plugin that could, for example, place an icon by the page to view all the images in an overlay (like the manipulation plugin).

We should determine whether this is in or out of scope for Diva 6 first release.