vtt support in Interactive Video

When creating an H5P "interactive video" the author shall be able to upload subtitles as vtt files, the user can turn them on or off and also switch to any language that's available.

For not cluttering the user interface, there will only be one additional button (e.g. labeled CC) that reveals a list of languages and the additional option "off" at the top. The list could use the same look and feel as the QualityChooser.