[videodb] adds UpdateLinkTable/UpdateActorLinkTable to video database
[videodb] adds UpdateDetailsForMovie() and calls it from JSON-RPC to more efficiently update movies.

Link tables are only updated if they need to be.
[videodb] only update art table if needed
[json-rpc] Pass an UPDATE_ITEM message rather than a general UPDATE message when a movie item has been updated over JSON-RPC.
Merge pull request #5170 from jmarshallnz/finer-grained-updates

Finer grained updates of video database