Actually, the menus themselves need to be in some kind of registry as
well.
This is essential if we really want to move to a fully modular system.
What happens now is that a package that wants to extend a menu or
keymapping (other than the World menu or the FileList) has to actually
override methods. This leads to conflicts (I keep getting my menu
changes clobbered by loading other packages).
We've attacked parts of this problem with different registries:
* the World menu
* the ParagraphEditor keymappings
* the FileList handlers
but we also need to do something about (for instance) menus.