Bug #1264, Revision 5 of interactive transcript JS feature

* Interim solution to select a timed-transcript <track> - srclang="en-GB-x-transcript"
* Added a "T" transcript button to player controls - TODO: CSS changes
* Adding some WAI-ARIA roles - "complementary" for the transcript, "search"..
* Bug fixes - config option "transcriptAutoScroll" etc.