Added support for generating thumb sequences (ThumbSequenceCreator)

This makes the thumbnailer plugin implement ThumbSequenceCreator instead of ThumbCreator to generate thumbs for multiple frames of a video file. It also implements some primitive runtime caching to avoid creating the same thumb again when the sequence "wraps around".

Also adds two new config options: The seek percentages of the individual frames, and the cache size:

Will be useful e.g. with my Dolphin MR and kio-extras patch, which add a slideshow of video frames when hovering a video file.

A similar MR was already incorporated into the original ffmpegthumbnailer. This one does the same thing with only minor changes.

I have opted for a KIO version check in the code to support this pending MR. That's just used for optimization and completely optional, so I didn't think bumping the required KIO version in the project was necessary.