[MAC] Fix toggle button

This PR fixes the ToggleButton backend on MAC:
* add missing OnToggled event
* use NSButtonType.PushOnPushOff as described in https://developer.apple.com/library/mac/Documentation/Cocoa/Conceptual/Button/Tasks/SettingButtonType.html#//apple_ref/doc/uid/20000085-79904
Typo here for untoggled.