This introduces the model-level support for menus, consisting of three

parts:


MenuItem: This is a model containing state for any kind of supported

  menu item (a standard action item, checkbox menu item, radio menu

  item, or a separator).


MenuItemsCollection: A collection of menu items, which will be used

  as the backing for a menu view.


MenuItemsRadioGroup: A radio group for tracking menu items, which

  ensures only one item can be marked as checked at a time.


The design for MenuItem consolidates all state for all types into one

model, rather than having subclasses for each type. There are a few

reasons for this:


It makes it easier to maintain items in a collection and still allow

   instantiation of new entries via the collection's .add().


It avoids needing to do instanceof checks for different operations.


It seems to be common practice (looking at the Apple, Windows, and

   GTK foundational classes for menu items).


The upcoming MenuView will make use of these classes to provide a full

menu implementation.

Unit tests pass.
Tested this thoroughly with the in-progress MenuView work.