WIP: Create thumbnails sequentially

When there is a file that requires its thumbnails to be created for the desktop, its fileItem object launches a new process, which runs the "createthumbnail.js" script without blocking the main loop. But when there are several thumbnails to be created, each process is launched at the same time. This can result in a load spike.

This patch fixes this by launching the processes sequentially, thus waiting until one process has ended before launching the next process. This ensures that there will be a limited number of process being launched and running simultaneously.