Enter key handling on collapses

[bkardell]

We are currently not handling the enter key to toggle on collapse afforandances (at least non-exclusive ones), @scottohara was suprised by that. It makes sense to me that it should, I kind of vaguely recall @jonathantneal having thoughts on this and maybe even introducing some code, so tagging him and creating an issue to track.

[jonathantneal]

If there’s an enter key, I expect it will behave like a button, and if it’s a button, then I expect the enter key and the space key will both work for toggling.

Additional notes:

• The enter key works on keydown.
• The space key works on keyup.