Collapse accessability

This addresses a decent amount of issue #13555.

- Added tabIndex aria attribute to collapse plugin
- Added tab-role, aria-selected, and aria-controls into the collapse html docs.
- Added support for toggling aria-expanded, aria-selected, and aria-hidden via the javascript collapse events.

A decent amount of the changes were inspired by the example at http://accessibility.athena-ict.com/aria/examples/tabpanel2.shtml
display:none in the .collapse CSS declaration already takes care of hiding the element from screen readers / AT. no need for explicit aria-hidden (though of course it doesn't do any harm, just over-specifying things)