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.