Add support for upnp:playbackCount and upnp:lastPlaybackPosition tags.
These are used by Kodi to keep track of bookmark information as well as
determining whether to show the checkmark to indicate that the video
has been played.
Also add support for the UpdateObject command, which Kodi uses to
update the playbackCount and lastPlaybackPosition information.
This change requires a DB schema update, which should be done
automatically on the first run.
Inspired by SF user Karsten's patch #167.
