Support native captions in html5 tech.

Don't auto-add the textTrackDisplay component.
Add a 'featuresNativeTrack'. Currently hardcoded to false for everything
except the html5 tech.
Add textTracks and addTextTrack methods to html5 tech.
Proxy player level methods to tech if we're using native tracks.
Fix up track menu items to work with spec tracks or vjs tracks.

This is the beginning of step 1 for the [revamp](https://github.com/videojs/video.js/issues/1655) of text tracks.