MediaFile thumbnail event hooks + VideoThumbnails plugin

The exception thrown from MediaFile will be caught and simply result in
no thumbnail at all right now. In the future we might use a catch-all
and have a "cannot generate preview"-icon or something.

VideoThumbnails requires php5-ffmpeg and php5-gd.

classes/File_thumbnail.php

@@ -84,6 +84,8 @@ class File_thumbnail extends Managed_DataObject
     /**
      * Save a thumbnail record for the referenced file record.
      *
+     * FIXME: Add error handling
+     *
      * @param int $file_id
      * @param string $url
      * @param int $width