No thumbnail display on multicanvased work if the canvas is missing a label

[markpbaggett]

I'm not sure if this is a bug, but I thought I'd report just in case you think it is.

If you have a multi-canvased manifest and the canvases don't have labels, there are no thumbnails directly under Viewer to aid in navigation. In the spirit of presentation 3, it seems like the viewer should generate these if missing.