Tabbed interface: improvements for screen readers

 1. Removed `enter` key behaviour for tabs as it was preventing a
    smooth transition to/from forms mode which resulted in an extra
	`enter` key press being required.
 2. Set `aria-live="off"` for the tablist.  This greatly reduces
    the amount of content read by screen readers when the arrow
    keys are used to change tabs or `escape` is used
    to stop the tab cycle.
 3. Set `aria-live="polite"` on the "Play" button.  This causes
    changes to this buttons text to be spoken.
 4. Backported `escape` key to stop tab rotation.

Discussion is in #1081.
Merge pull request #2920 from patheard/tabs-aria-live

Tabbed interface: improvements for screen readers