Keyboard Accessible MenuItems and VolumeMenuButton

Makes the volume control and menu items accessible via tab navigation on the keyboard.
Fix for #1517 
I'm a little hesitant on the .visuallyhidden idea (although I experimented with it and does work alright). My first concern is that we also need to listen to hover in code rather than css. Secondly, I kinda like being able to use the lock/unlockShowing methods on menu.

The width: 0 works a bit better. It winds up being more css because you have to explicitly style the vjs-menu-content as well as the vjs-menu. However, I think this might be a better solution. I'm gonna try to clean my experiments up and push them up.
 Sound good?