Follow-up r84395: Give MediaHandlers the option to remove items from the thumbnail...

Follow-up r84395: Give MediaHandlers the option to remove items from the thumbnail purge list. Is needed for TMH because thumbnail rendering may be very expensive for media files. Haven't thought yet of a way to integrate this with the UI, but the current framework should be flexible enough to implement that later.

Added array $options to File::purgeCache(), LocalFile::purgeCache(), LocalFile::purgeThumbnails(), ForeignAPIFile::purgeCache() and ForeignAPIFile::purgeThumbnails() which is currently empty, but can be used later to indicate a full or partial purge.
Added MediaHandler::filterThumbnailPurgeList(), which can remove items from the purge list and also gets passed this $options array