Make still-running background jobs non-blocking

[naglepuff]

Instead of waiting for the running job to complete before allowing the use to move to the next step, we could instead check for any annotations that have been spit out by that job, and move the use to the next step with only a few images at first.

We could potentially display a count of all images that are still being processed, and provide a mechanism to refresh the page/image list.

[cooperlab]

This is related to Issue #28. Perhaps a notification that tells the users that labeling can start once a slide is finished, and that additional slides will continue to process in the background (maybe where to check this too).

[manthey]

The superpixel algorithm will be able to report progress when DigitalSlideArchive/superpixel-classification#6 is merged. Currently, that would be the new default, so you get a progress toast in the bottom right corner show how the latest training is going. If we run jobs in the background, we might want to reposition this or not show it.