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.php

@@ -433,6 +433,11 @@ class File extends Managed_DataObject
         return File_thumbnail::getKV('file_id', $this->id);
     }
 
+    public function getPath()
+    {
+        return self::path($this->filename);
+    }
+
     /**
      * Blow the cache of notices that link to this URL
      *