Add a callback property that is called whenever the MenuItem focus is changed.

Sometimes it will be called when the focus isn't changed.  For example:
* If top menu item is keyboard focused (call mui#1) and then the up arrow is pressed (call mui#2).
* If some menu item is keyboard focused (call mui#1) and then that same menu item is clicked (call mui#2).

Unmounting the Menu component in order to cancel the FocusRipple.pulsate() timer.  Before returning, this timer callback always starts a new setTimeout() for itself.  This outstanding timer stopped Node.js from exiting.

Put the `event` argument at the front to be consistent with other functions.

Add a spec for selecting a MenuItem via hotkeys. The new spec uncovered two pre-existing bugs.