This adds the ability to show a kind of slideshow of thumbs when the user hovers a file item.
It revives the so far sadly neglected ThumbSequenceCreator in order to allow thumbnailer plugins to generate these sequences for arbitrary file types. The thumbs are loaded just-in-time when the user starts hovering. The switching interval is configurable in Dolphin. Since I'm not aware of any thumbnailer that implemented ThumbSequenceCreator, I have modified kffmpegthumbnailer to make it work for video files. These updates are now merged into ffmpegthumbnailer. Without it, the slide show will not be shown. With this modified kffmpegthumbnailer, the number and seek positions of the displayed frames are also configurable.
What do you think?