Add thunar_thumbnail_cache_delete_file(). Use queues for processing.

We again use something like the wait queue in the old ThunarThumbnailer
here for grouping Move() and Delete() calls. The same will be done for
Copy(). It performs really well, moving 200 images from one folder to
another only generates a single Move() request with all images. At least
on my fast machine.

The actual calls to thunar_thumbnail_cache_move_file() and
thunar_thumbnail_cache_delete_file() in the transfer and I/O jobs still
have to be added.