This introduces a new RB.MenuButtonView component, which uses the new

.rb-c-menu-button CSS component. This offers the ability to create a

single button that works as a drop-down menu, or a split button that has

a primary action and a separate drop-down button.
This is the successor to RB.SplitButtonView, which is now deprecated.

There are still differences, in that RB.SplitButtonView allows for

specifying IDs on menu items, but as that's not entirely useful in

practice, the functionality has not been carried over.
This improves upon RB.SplitButtonView in plenty of other ways. It can

now function in a split mode or in a single-button mode. It has ARIA

attributes and keyboard navigation (though RB.SplitButtonView now

inherits this). It also now smartly pops the menus either above or below

the button depending on space, rather than having to provide that detail

during construction.
Unit tests cover the majority of the rendering, accessibility, and

events.

Tested RB.MenuButtonView in single-button and split-button modes.
Tested all mouse and keyboard navigation.
Tested legacy usage of RB.SplitButtonView.
Checked the elements in the ARIA tree to make sure they appeared sane.
Unit tests pass.