[OMXPlayer] Pre-filter subtitles streams to relevant ones before sorting them by priority.
[OMXPlayer] Add option "Prefer external subtitles" to category subtitles and adapt DVDPlayer accordingly.
[OMXPlayer] Prefer forced subtitles if "original stream's language" is set as subtitle preference.
[OMXPlayer] Change forced subtitle when changing audio track.
[OMXPlayer] Add a doxygen comment to the class PredicateSubtitleFilter.
[OMXPlayer] Add a doxygen comment to the class PredicateSubtitlePriority