[videodb] Make it possible to retrieve empty TV shows internally.

This adds a boolean to GetTvShowsNav to allow retrieving of TV shows without
episodes even if videolibrary.hideemptyseries is enabled in
advancedsettings.xml.
[jsonrpc] Allow retrieving empty TV shows via VideoLibrary.GetTVShows.

This adds a boolean parameter ("includeempty") to VideoLibrary.GetTVShows to
allow retrieving TV shows even if they have no episodes and
videolibrary.hideemptyseries is true in advancedsettings.xml.