Pointer cursor on Tab widget

The cursor CSS property on top of the Tab widget is now set to `cursor` unless the tab is disabled.
It would perhaps be better to write `/Focused/.test(stateName)`, as stated in
[the MDN documentation](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/match)
> If you need to know if a string matches a regular expression `RegExp`, use `RegExp.test(str)`.