Divorced odid MediaServer from MediaFileItem/VideoItem/etc and MediaObject/MediaItem/MediaFileItem fixups

First implementation of ODID.MediaItem (subclassing Rygel.MediaItem)

Moved more item-specific logic from MediaFileItem to MediaItem.

Fixed up references in ODID MediaServer to use ODID.MediaItem instead
 of VideoItem.

TODO: odod testing, formatting pass