Use classes instead of inline styles for hiding / showing

I like that this makes the class usable for components that might exist outside of the player div. At the same time it reduces the specificity to where this could have no effect in potentially a lot of cases. I'm pretty sure vjs-hidden would have no effect if something was defined as such.

```
.vjs-default-skin .someComponent { display: block; }
```

I'm not totally sure if we should worry about that. Or maybe if we should use `!important` in this case.