Stops top menu item being triggered when opening the menu using the keyboard

Fixes https://github.com/videojs/video.js/issues/1446 

Key press handler is only active when either the MenuButton has focus or the Menu is open (allows closing the menu when a menu item is selected).
I'm having trouble following the logic here. It looks like the `onKeyPress` listener calls `pressButton` which then adds the `onKeyPress` listener. I think that might duplicate the listener.