This commit removes a large amount of redundant code that needed
to be kept in sync between platforms and also makes it much easier
to add new menu-related functionality since little to no platform
code needs to be altered anymore.
This commit also greatly improves code locality in context menu
handling by allowing context menu click handlers to be closures.
This commit temporarily introduces a SetMainMenu API, which is rather
hacky but only necessary until an abstraction for windows is added.
