#128 Process background and load images from Omero on separate threads to avoid blocking main

For this merge request, I did not implement the removal of the Apply button since I wouldn't have been able to finish implementing that in time.

I will not close issue #128 (closed) and will create a separate merge request specifically for that later. Also, ImageRepository is spaghetti, but it works. I'll clean that up also before closing the issue.

Created a separate ImageRepository with the intent of being the middleman for always making sure there is an image object for the rest of the software to work with - where possible try to avoid accessing that image via GelImage.image property and use ImageRepository.get(GelImage) instead. I'll be removing the GelImage.image property later.