Captions

This is the new all encompassing captions PR.
It's still have some issues, like `cuechange` firing unconditionally and some issues with the captions displaying but there won't be much architectural changes to this.

I'd like at some point to take all the TextTrack stuff out of videojs and include it as a dependency similar to vttjs.

Things left to do:
* [ ] A whole bunch more tests.
* [ ] `cuechange` needs to be updated to only fire once per change. It currently fires a couple of times per change.
* [ ] When adding a cue to a track programmatically, it needs to remove that cue from other text tracks if it existed in any other track.
* [ ] The menus don't work great on mobile devices. Particularly, with respect to hiding the menu after usage.
Another option here would be to have textTrackDisplay always be a child of player, and have it disable itself whenever nativeTextTracks are used. That might be a little cleaner than having a child of player inject a sibling. 

As is there might be a bug here. It doesn't look like textTrackDisplay gets removed when this tech is disposed, so the next tech will get loaded and add a second textTrackDisplay.