MenuElement functionality

Functions working on MenuElements:

• menuElement :: Picture -> a -> Component (MenuElement a, DisplayHandle) create an element with a Picture label and a value to output when selected.
• getSelection :: MenuElement a -> IO a get next selection of this element.
• setMenuElementLabel :: MenuElement a -> Picture -> IO () change the MenuElement's label.
• disableMenuElement :: MenuElement a -> IO () prevent interaction on element, indicate to user by greying out.
• enableMenuElement :: MenuElement a -> IO () enable interaction on a previously disabled element. Enabling an already ready element has no effect.
• nameMenuElement :: String -> MenuElement a -> MenuElement a Associate a name with the menu element.
• combineMenuElements :: [MenuElement a] -> IO (MenuElement a) Merge a set of MenuElements together, such that any selections made on the elements in the set is reported to the combined element. The combined menu element is `virtual', it does not have any appearance, although all menu element operations can be applied to it.
• mapMenuElement :: (a->b) -> MenuElement a -> MenuElement b Return a new MenuElement handle that for each value output on the handle passed in, output a new value by transforming the incoming.