Added support for fixed-length sequence loops to ThumbSequenceCreator.

This MR depends on this separate MR at kio, and should not be merged before the kio one is merged.

This adds some additional metadata to the thumbnail slave. See the other MR above for a detailed explanation.

Can someone please confirm that this way of setting KIO slave metadata is correct? It seems to work, but I want to be sure I didn't miss or misunderstand something.