[BUG] Some problems with subtitles on OGM files

[Tyr Antilles]

With Hybrid v.2018.05.21.dev3 most of the problems are gone, but SRT is included on the MKV output only if the file is transcoded when SRT is not already extracted on the output folder. If SRT is already there, the output MKV file don't have included in the output the subtitle.

Detailed steps:

- Having output folder empty, I loaded the OGM file on Hybrid, and started transcoding.

- Everything works great and the resulting MKV file plays well and have SRT included.
This is what media player reports of the file:
Video: HVC1 640x480 23.976fps [V: hevc main 10 L3.0, yuv420p10le, 640x480 [default,forced]]
Audio: AAC 44100Hz stereo [A: Japanese [jpn] (aac lc, 44100 Hz, stereo) [default]]
Subtitle: UTF-8 [S: English [eng] (subrip)]
Subtitle: UTF-8 [S: No subtitles]

- If I do not clear the output folder and add the OGM file again on Hybrid, I see the subtitle is not automatically added on subtitle queue. If I proceed to transcode the file anyway, the result look like this:
Video: HVC1 640x480 23.976fps [V: hevc main 10 L3.0, yuv420p10le, 640x480 [default,forced]]
Audio: AAC 44100Hz stereo [A: Japanese [jpn] (aac lc, 44100 Hz, stereo) [default]]

Of course, I can manually add the SRT file that already exist there.

I think something is still off when the subtitle is already extracted in the subtitle queue. All other problems including the crash are now gone.