Issue #160: Refactor watched status indicators

* Move main logic to video content layer
* Toggling now handled at the video level. Reduces code duplication.
* Series content contains logic for watched status.
* Movies and Episodes now follow Plex logic for watched status
* Video Playback with internal player will now mark as Watched when more
  than 90% of video has been completed.  This aligns with offical plex clients