Some MediaHandler subclasses were setting custom properties on the File
object in order to cache file-associated state. So:
* Add File::getHandlerState() and File::setHandlerState().
* Put them in an interface, which will be used in a subsequent commit in
MediaHandler::getSizeAndMetadata().
* Use them in DjvuHandler.
* Provide a trivial implementation of the interface, for use in testing
and in the subsequent commit.
Change-Id: Ic365384ff13f7898c1203da38c4405abf03d7563
