One minor bug fixed, one minor perf optimization

1. Right now the shadows over the thumbnails are making the thumbs underneath them non-clickable. Adding `pointer-events: none` fixes this problem.

2. In webkits there is a small lag on scrolling into the slideshow, when the transition for arrows and icons happens during the scrolling. Using individual props instead of `all` makes things better in that place (not ideal, but noticeable on my macbook). This adds 100 bytes to the CSS, I think it's fair price for this small perf. boost (and if you'd use `.3s` instead of `300ms`, you'd get only 84 extra bytes here, haha).