Add support to AD functionality in the player - #916

Related issue: #839
Related issue: #850

Changes in this PR:

• extract audio description text-tracks from parsed supplementing annotations by detecting the /descriptions text in the annotation id
• build the parsed audio description text-tracks into the <tracks> in the player element
• create a custom button for AD and enable it in the player's control-bar when there are audio description text-tracks
• use cuechange event handler of the active descriptions text-track to read the cues using WebAPI speechSynthesis

Implementation notes for the AD button:

• VideoJS' native AD button: this renders in the player's control-bar and is only enabled when captions are turned OFF. This takes away the user control for the button, and limits captions and AD tracks operations. I was not able to enable it programmatically in the Ramp code once the player finished loading.
• A fully custom AD button: build a fully custom AD button without feeding the audio description text-tracks to VideoJS. This work seamlessly with what Ramp needs, i.e. a toggle button with speech synthesis. However,
  this needs Ramp to parse and synchronize the cues with the player's timeupdate event. This makes the code more complex and could make the event synchronization for different browsers difficult (as we've seen in the past).
• Implemented behavior: a hybrid solution between VideoJS' native behavior and a custom component implmentation. This feeds audio description text-tracks to VideoJS and build a custom VideoJS component as a toggle button. This way Ramp doesn't need to parse and synchronize the cues in the AD text-tracks, but can tag onto the cuechange event emitted by the active text-track marked by VideoJS to enable speech synthesis. This doesn't make the AD functionality dependent on the captions like the native AD functionality.

Notes:

• AD button is only added to a Video player's control-bar when there is an audio description text-track given as a supplementing Annotation.
• AD button is a toggle button, and is turned OFF by default. i.e. turned OFF on page load and preserved across Canvas changes in the same Manifest.