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).
We have to readd the listener to get bubble key press events from the menu item so we can close the menu.  Ideally we would have the menu trigger an event when it is activated and deactivated.

If you look further down the function the first menu item is focused on, this will remove the menubutton's keyup listener set in the button focus listeners.  Looking at the code now, it might be better to move this keyup listener to inside that if statement, so it is only set if there are menu items.