Add "Subtitle settings" to "Setting"

[oas777]

This is a request for the next sprint: Other video players have the option to customise the subtitles:

(YouTube, too much detail)

(Vimeo, just about right)

(video.js, too little information)

I think we need something along those lines as well to change the custom settings.

[oas777]

Submitted as feature request https://github.com/polimediaupv/paella-core/issues/349.