Communicate to the user when segmentation series is being loaded #687

When I open this study https://volview.kitware.app/?urls=[s3://idc-open-data/7db6c700-b92c-4062-a859-1dda082f871d,s3://idc-open-data/647238d1-9e55-4755-9b55-9d98d7193426,s3://idc-open-data/bae5a681-6ec2-4994-a3d8-77578bca2dc9,s3://idc-open-data/4fc888b5-89db-47b0-9b11-acac24e50f1a,s3://idc-open-data/dd64b26c-c515-46dc-8fd8-ba45afd5d4e7], CT is opened automatically, and SEG is opened automatically too, but there is a delay of >25 seconds between seeing the CT image and seeing SEG. In the meantime, the UI is fully responsive, and there is no indication something is being loaded. In fact, first time I tried it, I double-clicked SEG and then selected "Add as layer" for the SEG series, in the course of that 25+ sec wait.

Would it be possible to add some kind of a persistent indicator that SEG series is loaded? Something like "Loading ..." text over the SEG series thumbnail, or a spinner over that thumbnail, would be quite helpful!