A MenuItem that as the name suggests allows for a horizontal Separator to be embedded within it,
by assigning a Separator to the content property of the CustomMenuItem
This is provided for convenience as groups of menuitems can be separated
by a separator. Instead of a creating a CustomMenuItem for this purpose, the user
can use this class as indicated below.
MenuItem item1 = new MenuItem("Item1");
MenuItem item2 = new MenuItem("Item2");
SeparatorMenuItem separator = new SeparatorMenuItem();
Menu menu = new Menu("Items");
menu.getItems().addAll(item1, separator, item2);
MenuBar menuBar = new MenuBar(menu);