EogWindow: Raise priority of image loading and transformation jobs
That should prevent image switching/loading or rotation getting starved by queued thumbnailing jobs.
By having the same priority they could delay loading a new image until all visible thumbnails in the collection had been created. Especially on slow disks (like network shares or slow SD cards) this could make image switching impossible for quite some time.
The same could happen with transform jobs. Rotation of the first shown image could be delayed until the thumbnails were finished.
See #305.