Enable expanding of section headings via space and enter keys in stable

For accessibility purposes.
See:
http://www.marcozehe.de/2013/04/24/easy-aria-tip-6-making-clickables-accessible/

This has been tested in beta for a good period of time, so now
pushing to stable.

Change-Id: I8a8c203ae443c90b5c8948414688c7fb996bf67c
Merge "Enable expanding of section headings via space and enter keys in stable"