Actual sane support for TV show statuses.

Although it was committed in 2007 when adding support for TV shows, statuses
were never actually supported.  As of now scrapers can write anything in it, and
it's only accessible via smart playlists - there's no support for it in the
media filter and skins can't access it.

Even though this commit doesn't implement the logic for the media filter, it
generalizes support for it, by defining specific integer values for the various
statuses and exposes it to skins (ListItem.TVShowStatus).

It currently only supports values provided by TheTVDB API:
  * Continuing (i.e., active; 1);
  * Ended (2).
json-rpc: Retrieve/set "status" in VideoLibrary.Get/SetTVShowDetails.
[metadata.tvdb.com] Retrieve TV show status.
[confluence] Show TV show status in video library and DialogVideoInfo.