return index to consumer by onClick

[KathyWang0208]

Here is a case: we want to pass the specific image when user click one image in image gallery to achieve the aim which user can see the image in image gallery wall. so we need render image item as a button. one problem with this is that image-gallery-slide is clickable, and the internal image item also clickable, the behavior is not expected by a11y rules. so make this change to fix this issue.

[xiaolin]

if you can update the doc I think this is mergeable