This allows the user to display multiple tracks when
`allowMultipleShowingTracks` is passed to the `TextTrackDisplay`.
Currently, multiple tracks must be shown programmatically and cannot be
done via the subtitles menus.
In addition, this adds two new classes to cue elements:
`vjs-text-track-cue` and `vjs-text-track-cue-${track.language}`. This
allows easier targetting with CSS.
Example usage:
```js
var player = videojs('example-video', {
textTrackDisplay: {
allowMultipleShowingTracks: true
}
});
```
Fixes#5798.
