Move subtitle addon core handling to core.

This is primarily aimed at taking away some of the load from @amet at having to handle 1249591 subtitle services.

It moves the UI portion of the xbmc subtitles script into core, and has the services run as plugins from there.  This will allow (in future) for better integration of downloading of subtitles.  For now it's still a little clunky in that downloading of subtitles is still somewhat separate from embedded or external subs already on disk. Nonetheless, it's a starting point and IMO important to get that starting point in so that the subtitle service guys can get things done (it's a minor change for them) and so that skinners can get their changes done.

If anyone has any ideas on how it could be better integrated in the future, then that's cool - please discuss on forums to keep this focused on code discussion.

It does add a new window for skinners to deal with, but most already have it in place as most support the subtitles script.  Their existing window can be transferred over just by renaming the file in most cases.

@Montellese: It relies on multi-select list settings, so a solution to that needs to happen first.  Note that all it needs is a comma separated list of languages, so we don't actually need any special retrieving of a multi-select setting pre-broken down (as we'll just stuff it back together again).