[ext. subs] Check for delimiters (Ticket #15011)

This fixes trac ticket http://trac.xbmc.org/ticket/15011
Only https://github.com/ace20022/xbmc/commit/d4d0f4b4781adfdc392f3416a80e22aff61b2509 is relevant the other are skipped after https://github.com/xbmc/xbmc/pull/5204 got merged.
@Voyager1 @Karlson2k for review
@tamland - One of these delimiters - or "." + the file extension - has to immediately follow the base movie name. So Movie.mkv and Movie.srt would be OK, but Movie.mkv and MovieABC.srt would NOT be OK. However Movie.mkv and Movie.ABC.srt would again be OK. Hope this helps.