Internal data structures

• long menuID - this may be used to store the ID of any window-library-specific menu structure. It is a public variable which is not used for any other reason by PopupMenu.
• int menuExists - another public variable which may be used by the winding library if necessary, to indicate the presence of a menu structure.
• static int nextReturnCode - the next return code to assign to an item. This must be unique, and greater than zero. It is static so that all items across all the PopupMenu instances can get unique return codes.
• ResizeArray<PopupMenuItem *> menuItems - the list of the menu items in the menu. Each PopupMenuItem object contains the name and information for a single item, be it a regular item, a separator, or a submenu.
• char *menuName - the name of this menu.