reworked the fetching of the caliper images

props to @ewels for both the idea and the great help in the implementation.
This will make the caliper images loaded by one after the other, only if someone is looking at them, and without interfering with the sample info.

Didn't manage to make it work, and it will have atime on screen of a few seconds at best. If I manage to find how it works, why not :p