[Fix] Fix several bugs regarding visibility of subtitles

This PR fixes 

- http://trac.xbmc.org/ticket/14821
- http://trac.xbmc.org/ticket/14864
- visibility of external subtitles added between playback of a file without (internal) subtitles.

It replaces pr https://github.com/xbmc/xbmc/pull/4089 and pr https://github.com/xbmc/xbmc/pull/4084