Keyboard Accessible MenuItems and VolumeMenuButton

Makes the volume control and menu items accessible via tab navigation on the keyboard.
Fix for #1517 
I think we could use the [hide-visually](https://github.com/videojs/video.js/blob/a2710b60f1316635fddb34c2ca26d6feb691a73c/src/css/video-js.less#L1045) mix-in here instead of z-index. There's potential that a menu button component could be used somewhere where the menu wouldn't be covered by the the player.