Video Subtitles

As a Web UI users with read permission on a video with subtitles
I want to see the subtitles
So that I get a written support  

Functional description

Today, we do not support subtitles on video content.  We should add this support.
To this end, we need to

• store the VTT files along with there language.
• Generate dynamic renditions for each available language
• Expose all vtt's files as <source/> (rendition) on https://github.com/nuxeo/nuxeo-ui-elements/blob/master/viewers/nuxeo-video-viewer.js along with their language.
   

possible solution

(this is just a proposal)
Add a new schema on video : multivalue complex with a blob and a string for the language code.
https://github.com/nuxeo/nuxeo/blob/master/addons/nuxeo-platform-video/nuxeo-platform-video-core/src/main/resources/schemas/video.xsd