Improve accessibility support in non-Firefox

Only add tabIndex to the scroller in Firefox since setting it prevents
screen readers from seeing the table rows in other browsers.
CHANGES.md: Note non-FF a11y fix
Fix #769: OnDemandList: only add loading node when running a query
CHANGES.md: Note OnDemandList loadingMessage regression fix
Fix #751: pass cell's value to canEdit when using a shared editor
editor: canEdit unit test clean-up

* Add descriptions to assert calls
* Use edit API instead of emitting fake click events
CHANGES.md: Note editor canEdit value argument fix