Make slideshows keyboard accessible

Hello, here's a quick pull request for issue #1001. As suggested in that thread I have set the slideshow controls to default to `opacity: 0.5` rather that `display: none`. The controls fade to full opacity on focus or hover, and I have added checks so that they won't fade back to 0.5 as long as either the mouse is over the slideshow or the controls are focused.
Is strict comparison merited here?  Wouldn't it be better to do `jQuery( document.activeElement.parentNode ).hasClass( 'slideshow-controls' )` instead?