[Accessibility] Implement SplitMenuButton

Implement SplitMenuButton on Mac and Window

Windows there is a SliptButtonControl type, which requires IInvokeProvider and IExpandCollpaseProvider.
On Mac there is not a one-to-one, I did find a SplitButton on Mail (the button to add flags to emails)
It is implemented using a AXButton for the left side, and AXMenuButton for the right side.