PopupMenus can now include separator rows

PopupMenu dropdowns should support separator rows so developers can visually group related options. Today a PopupMenu can only add text rows, so lists that logically group options (for example Small, separator, Large) have no way to show a dividing line. Adding PopupMenu.AddSeparator() lets developers append a non-selectable separator row that occupies an index and counts toward RowCount, but cannot be selected by mouse, keyboard, or code.