This extends the button styles to enable support for segmented buttons.

These are elements that look like a single button, but have several

independent components, such as a separate dropdown arrow that behaves

differently from the main button label. Each segment has its own

background, for hover/click effects. They're all still bound to the

visual style of the outer button, though.
This is similar to SplitButtonView, and in fact will be used as the

base for that in an upcoming change in order to simplify things and

create a better visual consistency.

Tested this along with an upcoming change for SplitButtonView. The

styles were tested on macOS (Chrome, Firefox, Safari) and on Windows

(IE, Edge, Firefox).
Made sure all existing button styles still looked as they did before.