Fix several issues with the non-destructive rescan functionality.
Most of these issues also affected inotify scanning as well. These
include annoying debug messages, adding album art for files that we
aren't supposed to be scanning anyway, incrementing the UpdateID when no
changes were made to the database, and other smaller issues.
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.
