support for dual subtitles

I wanted to add such a new feature to vlc player, that the user can use a second subtitle from the list of available subtitles, and configure its position.

The reason is that I often need to use two subtitles when watch video, and I found other users have that that needs, too. Although I can merge subtitle files manually, but that's cumbersome, specially sometimes the subtitles are embedded in the video container. I would like to do it myself and I'm reading source.