Fix rjsf-team#365: Tab stop for Add button + (rjsf-team#392)

Author: eeinowski

src/components/fields/ArrayField.js

@@ -419,7 +419,7 @@ function AddButton({onClick, disabled}) {
  <div className="row">
  <p className="col-xs-3 col-xs-offset-9 array-item-add text-right">
  <IconBtn type="info" icon="plus" className="btn-add col-xs-12"
- tabIndex="-1" onClick={onClick}
+ tabIndex="0" onClick={onClick}
  disabled={disabled}/>
  </p>
  </div>