[videodb] add routines for creating link indices
[videodb] refactors UpdateActorLinkTable into UpdateActorLinksToItem, AddActorLinksToItem  TODO: Add usage of it (note no musicvideo yet, though we could special-case?)
[videodb] switch actor and actor link tables to the new layout
utilise AddActorLinksToItem, dropping AddDirectoryToItem, AddWriterToItem, AddArtistToMusicVideo
squash into director redo (drops it from Addgenredirectorstudio
cleanup AddCast to drop unused param
drops AddActor|Writer|Director|ArtistToItem()
[videodb] switch the studio and link tables to the new format
[videodb] switch the genre and link tables to the new layout
[videodb] switch the country and link tables to the new layout
[videodb] switch the tag and link tables to the new layout
[videodb] updates UpdateLinkTable() with the necessary changes for genre, country, studio etc.
[videodb] cleanup GetCommonNav to use new link setup
[videodb] tags can be updated using UpdateLinkTable
[videodb] refactor UpdateLinkTable into AddLinksToItem, UpdateLinksToItem
[videodb] cleanup genre, country, studio additions
drops addgenredirectorstudio
drops AddStudioToItem
use AddLinksToItem for tags
[videodb] simplify the AddLinkToActor/AddToLinkTable/RemoveFromLinkTable functions