[videodb] factor out the addition of paths to the tvshow link table
[videodb] move parent path id for shows to tvshowlinkpath
[videodb] drop the basepath for tvshows, as it's the same as the path with the exception of multipaths
[videodb] allow AddTvShow() to take in an existing show ID so it just adds a link to tvshowlinkpath
[videodb] remove duplicate tvshow rows in favour of multiple links in tvshowlinkpath
[videodb] search for existing tvshows that match rather than adding a duplicate when adding the same show with a different path
[videodb] if we have multiple links from tvshow to path, we need to be careful with the grouping in the tvshowview.
[videodb] drop GetStackedTvShowList and Stack as they're no longer required