Pause auto play when hovering over a gallery item

This pull request pauses the auto play feature when hovering on a gallery item as you'd expect from a javascript carousel.

Unfortunately this wasn't as simple as utilising [animation-play-state](https://developer.mozilla.org/en-US/docs/Web/CSS/animation-play-state) because it doesn't pause animations that are big delayed. This causes animations to start queuing once their delay is over.

To get around this limitation I've programmatically generated an animation for each animatable element with the keyframes that simulate a delay. Sadly not as elegant, but nifty.