Keyboard Accessible MenuItems and VolumeMenuButton

Makes the volume control and menu items accessible via tab navigation on the keyboard.
Fix for #1517 
The .vjs-default-skin .vjs-menu selector is going back to using display: none based on your previous comment. So I don't it should be using hide-visually. 
I don't think it makes sense to use visually-hidden for the volume bar, because then we have to manually undo all of the styles it entails and repeat styles we've already created.
Thoughts?