BaseMenuHandleView is a new base class responsible for setting up a

MenuView and opening/closing it based on key and mouse events. It

provides the base interface for standard options, a buildMenu() method

for setting up the menu and event handlers, and subcomponent handlers

for crafting menu items directly in the handle.
MenuButtonView has been updated to leverage this, allowing it to

concentrate on the button states and rendering, rather than the menu

management.
An upcoming change will introduce a new component leveraging this base

class to provide a label-based drop-down, which will be useful for more

menubar-style UIs.

Unit tests passed.
Thoroughly tested MenuButtonView in Storybook and in Review Board.