Add bespoke-visited class to any slide already visited

[mojavelinux]

It's typical to want interactions (such as fragments, bullets or animations) to only play the first time the slide is seen. If you return to the slide, you probably want it to show a fixed state (perhaps the final state). To allow plugins to respond to this status, slides that have already been visited should be marked as such using a CSS class.

I propose that the class bespoke-visited (or bespoke-seen) be added to any slide that has been previously viewed. I think the appropriate time to add the class is when the slide is being deactivated.

Naturally, what happens as a result of adding this class is up to the plugin author or presentation author (the beauty of Bespoke's modularity).