HTML5 fix: Make every cell keyboard-focusable as per W3C spec ( https://html.spec.whatwg.org/#focus-management-apis ); without us setting a valid `tabindex` DOM node attribute any `cfg.takeFocus=true` config option for `setActiveCell()` et al will fail to deliver in Chrome 38.x and upwards, at least, as calling `.focus()` on a node which doesn't have this apparently keeps the focus (`document.activeElement`) stuck at BODY level :-(  -- also added assertions everywhere in the code to catch this problem when it's recurring.