Improve handling of missing slide label and overview images

[fedorov]

Suggested by @ulrikew

I have a feature request for the Slim viewer: Can the empty space for slide label be removed or be filled with a dummy image that states Not available or something similar?

Another feature request: Can the same be done for the thumbnail images on the left side that allow users to go to the other slides of the same patient? We have a lot of space that doesn't serve any purpose. All new CCDI MCI images will lack label and thumbnail images and this problem will persist.