Introduction of a hook_file_unmanaged_delete() to invoke actions when files are deleted.

A hook_file_unmanaged_delete($patj) will make it possible to do actions when an unmanaged file has been deleted successfully.

One good example could be when flushing the image style cache, for a site running behind a reverse proxy like varnish. This would normally require the varnish cache to be manually cleared, or waiting for the cached responses to expire, but when we can log the files that are being deleted, we are able to send a list of paths to the varnish, so the items can be purged. I guess you can find tons of other situations where this could be useful...