Text Management refactoring

Created by: jeremco

Hi

This pull request contains some refactoring to have a better comprehension on how subtitles and captions are working on dash.js. One new text controller is added to manage tracks. The way source buffer controller is created is hidden by a text buffer controller proxy. The text source buffer is dedicated to manage and create cues. The code relative to text is then centralized and is more easy to understand. Each 'class' has now its own responsability

Jérémie