Use CSS to show and hide components. Use classList when supported.

Instead of setting `display: none` directly on a component element to hide or show it, use vjs-hidden. This allows whether to show or hide elements to be overridden in custom skins or plugins. For example, a social plugin might want to show the loading spinner while it loads a login pop-up even though video.js would probably be hiding the spinner by default during that time. Added a load-time check for the classList property on elements so the implementation can avoid manually parsing the class property if it's present.