Improve archive image thumbnailing

[ahodesuka]

The current way thumbnails are generated for archives, if I remember correctly, is that every file is decompressed at the same time (up to the thread limit set by the imagelist) and the thumbnail is created.
This causes an extreme IO bottleneck and most likely hinders performance.

The images should be extracted on one thread and there should be another thread waiting on a queue to create thumbnails for newly extracted images.