Port `addClass` and `removeClass` methods from core to remove reliance on `classList` API

This fixes the quality menu setup routine causing issues on IE <10 due to lack of `classList` support. 

Ideally, the quality menu would extend the `vjs.Menu`, 'vjs.MenuButton' and 'vjs.MenuItem' classes like text tracks do. This way, we could take advantage of each component inheriting `addClass` and `removeClass` methods, plus the built-in keyboard support and ARIA role-handling that these classes offer. 

A candidate for a future rewrite, perhaps? I'm definitely lacking the free time to do that at the moment, hence this quick-fix patch :wink: