Enable HTML5 video track support

This patch enables support for the HTML5 video track standard
which defines subtitles and captions for HTML5 video.

The HTML5 video controls will now have a popup button when
captions are available that allows the user to select the
preferred caption or turn them off.

Change-Id: Id5b837d1a7b536935cd5038812594b11c48c3480
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>