Image preview: Preload only thumbnails of neighbouring files

Currently, when you have a small zoom-factor, so that many files are visible in a folder (e.g. via compact view), then for all visible files xx-large thumbnails are requested at once, which might take few seconds (depending on the hardware,etc.) Only after that job finished, the preview will be updated. So when you first enter such a folder, you might need to wait longer than usual for the preview (tumbler might be on 100% CPU load for that time), which is not so nice.

A more gentle way to preload preview thumbnails might be, to only preload the preview thumbnails of all neighboring files. So for list-view, the previous and the next file. For icon view additionally the top and the buttom file.

Like that, only up to five xx-large thumbnails need to be requested at a time, while scrolling through pictures via keyboard will still be fluent / without the need to wait for the preview on each picture.