Change menus to use "disable" instead of "disabled" for the property name

Justification: The MenuItem API does not inherit from Node (it inherits directly from Object). MenuItem (and therefore all subclasses - Menu, RadioMenuItem, CheckMenuItem, CustomMenuItem) has a 'disabled' property that, whilst arguably more readable, is totally inconsistent with Node. This is because Node.disabled indicates that not just the node, but it's entire hierarchy is disabled. At present MenuItem does not have this notion - you can not disable a Menu and have all of it's MenuItem children become disabled. However, should we ever support this in the future (and it seems like a reasonable feature), we do not want to have trampled on disabled. Using disable instead would make us consistent with Node.disable.