[LLPlayer] Enhance automatic loading of external subtitles with language codes

✅**1. Enhance auto adding of external subtitles** Now external subtitles added automatically if it has exactly same name as video file, but subtitle filenames often has could contain language codes. Especially when it comes to adding 2 or more subtitles for single video. Example: `video.mp4` - video `video.en.srt` - subtitles file which indicate that it is English language `video.es.srt` - subtitles file which indicate that it is Spanish language and so on. In VLC or mpv subtitles with such filenames would be auto added. I wish LLPlayer could do that too. **2. Hyphen instead of blank** When two subtitle tracks not match it's cues exactly the subtitles are bouncing up and down the screen. I suggest displaying a hyphen (or something similar) instead of blank to preserve position. How is it now:  How I suggest: