Add support for a central subtitle folder.
The folder can be used to store subtitle files that Showtime can use
when playing a movie.
To configure path to the folder, open Settings -> Subtitles
Rework subtitle provider ordering
It's now possible for the user to freely select the order in which
subtitles should appear.
It's also possible to select on a per-provider base if the provider
should be candidate for automatic selection by Showtime itself