Extend videodatabase GetTvShowInfo api

Add an optional FileItem *, this allows the retrieval of all the
properties from a tvshow, as some are only set when a fileitem is
available.

This allows the JSON-RPC GetTVShowDetails to return watchedepisodes, and
fixes this bug:
http://forum.xbmc.org/showthread.php?tid=191142
Merge pull request #4561 from cg110/json_tvshows_watchedepisodes

Extend videodatabase GetTvShowInfo api