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.
Merge pull request #2931 from patheard/tabs-play

Tabbed interface: fix enter key blocking on tabs