Accessibility improvements

Added WAI-ARIA properties to implement the
Tab Panel widget design pattern.
http://www.w3.org/TR/wai-aria-practices/#tabpanel

Improved tab focus management so that only focusable elements within
the current slide can receive keyboard focus.

If the document focus is either a menu item in the controlNav or a
descendant of the slider but neither a descendant of the current slide
nor the previous or next button, we set focus on the currentNav item.

Fixed :focus outline property so that navigation links display keyboard
focus.