subtitle addons restructuring(retry of #1444)

- xbmc.python.subtitles hidden in addons manager
- create xbmc.subtitle.module so that subtitle addon services can be installed as modules

as in https://github.com/xbmc/xbmc/pull/1444, I cant amend there so this is a new PR.

nothing has changed since as i am not sure what direction is wanted/needed, this will greatly help in maintaining and keeping update intervals to a minimum and only to a needed services.

thoughts?