Added accessibility (ARIA) tagging

Bit more refactoring to support accessibility:
- Gave the accordion a role of "tablist" and tagged it appropriately per
WAI-ARIA guidelines for accordion elements
- To support this tagging, refactored all the HTML-specific open/close
state changes into SetAccordionElement* helper functions
- Passing both the content AND its corresponding header as a unified
section object, in order to ensure that the header element is correctly
updated when opening/closing animations complete
- Added keyboard support, including proper tab indices for the accordion
tabs
- Added support for collapsing the opened section by clicking on its tab