Keyboard Accessible MenuItems and VolumeMenuButton

Makes the volume control and menu items accessible via tab navigation on the keyboard.
Fix for #1517 
Yeah, it doesn't make sense to use it on .vjs-menu with the other change.

I was mainly looking for an alternative to z-index for hiding it. Would height:0 or width: 0 work? That seems to basically be what youtube is doing. Alternatively we could add a `.visuallyhidden` class that contains the hide-visually styles, and add/remove that class to/from the volume slider when needed.