When a folder is opened, display placeholder thumbnails _before_ attempting to look up pre-generated thumbnails.

Other information:
When a folder with many media file or photos is opened, if these files are created at different times and have very different names then thumbnails load very slowly and in random order. Icons that have not loaded their thumbnails are not displayed and when they start to be displayed, they are inserted into the already displayed icons changing their order and location on screen.

This causes other icons to move while the folder is loading. If a user opens a folder and then tries to immediately open a file, he is doomed to fail, because the files move around for the first few seconds. This does not happen if all the files in the folder do not have a previously generated thumbnail.

The problem is made even worse if there is a large number of pre-generated thumbnails (I have over 6000 files in ~/.thumbnails) as that slows down lookups. Slow harddrives (laptops) and high load on the harddrives (like when starting applications soon after system startup) is also making this worse. In worst case scenario I have seen icons moving around for up to 10 seconds after the folder window is displayed.