[Feature] add subtitle support

[medarob]

Hi,

it seems that webm does not support embeded subtitles into a webm file. Therefor you have to use external subtitles which can be added to a <video>-tag with a <track>-tag.

Is it possible to add subtitle support for videos?
Info: https://developer.mozilla.org/en-US/docs/Web/Guide/Audio_and_video_delivery/Adding_captions_and_subtitles_to_HTML5_video

For example, save the subtitle in the same folder (or a subfolder which has to be configured) and load it into the code. The same function you use to add other video formats like webm, ogg.

Only questions remain:

How do you add multiple subtitles for different languages?

This would be a solution:
clip-name.en.vtt
clip-name.de.vtt
clip-name.fr.vtt

How to define the default subtitle?
There exists a default attribut which indicats to the browser that this is the default subtitle file definition. How to define that so that it works for multilanguage?

I thought about adding the default option to the filename to indicate that this is the default subtitle, like this clip-name.de.default.vtt. Problem: Does this work when the CE was translated into another language? On a French website the default subtitle schould be in French as well. (Of course the user can change the subtitle in the frontend, so maybe that would be a minor issue.)

A better way would be to list all available subtitles for a videofile in the backend and the editor can choose the default subtitle. But not sure if that is possible?