Making the X larger

This was originally marked as wontfix in https://plugins.trac.wordpress.org/ticket/1693

However, I'd like to revisit it again. I think making the X larger makes it an easier target and less frustrating to hit (yes, I know you can hit anywhere and it'll close the Carousel, but not everyone knows that so users may be trying to click the tiny little X). We also had a request to make it larger for mobile viewing (on an iPad), here: http://wordpress.org/support/topic/making-x-on-carousel-a-little-larger?replies=1

I think it doesn't do much harm to make the X any larger, hence my request to consider this PR once again.

If you want to tweak the numbers somewhat, feel free!