Multimedia player: Added label and srclang attributes to caption tracks in working examples (fixes #2935).
Merge pull request #2936 from pjackson28/updatemplayerexamples

Multimedia player: Added label and srclang attributes to caption tracks in working examples (fixes #2935)
Multimedia player: Corrected incorrect data-type on English captions and indenting issue in French.
Merge pull request #2938 from pjackson28/mplayerfixes

Multimedia player: Corrected incorrect data-type on English captions and indenting issue in French
Tabbed interface: fix enter key blocking on tabs

1. `e.preventDefault()` has been removed from the tab's keydown event
   handler.  This allows enter key presses to pass through to screen
   readers that don't implement their own enter key behaviour.
2. The play button only has `aria-live="polite"` set when it has
   focus.  This prevents extra speaking of its changed text.