when focusOnChange is false -> remove attr tabindex

[alexandrubotez]

When tabbing through the slides, they have tabindex 0 even though focusOnChange is false.
The issue appears from initADA function where the slides that are in view are given the value 0 for tabidex.

I have added an if for the verification of the focusOnChange option.

screenshot here

     if (_.options.focusOnChange) {
           _.$slides.eq(i).attr({'tabindex': '0'});
         } else {
           _.$slides.eq(i).removeAttr('tabindex');
         }

I believe this is the only thing that is missing from #3032

Issue here.

[leggomuhgreggo]

Nice, thanks!