Improve subtiles logic

[Giovanni Musto]

To be implemented after #235 and #875.

Once the audio track selection and the "only forced subtiles" option are implemented, another feature could be included: an option (enabled only when "only forced subtitles" is enabled) that allow non-forced subtitles to be burned if there's no audio track for the specified language. I'll make a few examples:

File 1:

Audio: Ita, Eng

Subtitles: Ita

File 2:

Audio: Ita, Eng

Subtitles: Ita (forced), Ita, Eng

File 3:

Audio: Eng, Rus

Subtitles: Eng (forced), Eng, Ita (forced), Ita

Case 1:

Default audio language: Italian

Burn only forced subtitles: Enabled

Override "forced only" option if there's no audio track for specified language: Disabled

In this case file 1 will have no subtitles at all, file 2 the forced subtitles and the file 3 the forced subtitles of the preferred subtitle language.

Case 2:

Default audio language: Italian

Burn only forced subtitles: Enabled

Override "forced only" option if there's no audio track for specified language: Enabled

In this case file 1 will still have no subtitles, file 2 the forced subtitles and the file 3 will now have the non-forced subtitles of the preferred subtitle language.

In this way an Italian user (for example) can avoid unnecessary subtitles when the audio is Italian and have full subtitles when the audio is in another language. Thank you in advance.

[Petr Nejedly]

I would see this more in a conjunction with #730 (or as well). Something like:

[x] Show subtitles only when audio language doesn't match

This would serve subtitles when audio doesn't exist for the preferred language, thus removing the duplication.

[Giovanni Musto]

The purpose is exactly that, to provide subtitles only if there's no audio track for that language, but forced subtitles do not count, as they're needed even if there's the correct audio track.

[Petr Nejedly]

Thanks. I will leave this out of 1.7 for now to see how the audio tracks selection works first. Then we can build on it.