[RFC] Secondary subs

[dskiba]

Hello guys, first of all, thank you for creating such a useful product!

I’d like to discuss an additional feature: “secondary subtitles.” Personally, I find it very useful, as it allows me to get both language contexts simultaneously.

Thanks to the great architecture, it’s easy to implement, as I have done in WIP PR

In simple terms, it works like this:

1. Save the previously selected language subtitles.
2. If there is a previously selected subtitle, display it at the top of the page.

Bonus point: I think it's going to work for all supported platforms

The code is still a bit messy, but it works for me, though I’ve temporarily removed the draggable feature since I need to adjust the subtitles’ placement.

If you’re open to this feature, we can dive deeper into the details. But first, I’d love to hear the authors’ thoughts on it.

Let me know if you need further refinement!

Pull Request: #258