<md-sub-menu>
Menus display a list of choices on a temporary surface.
Menu items are the selectable choices within the menu. Menu items must
implement the Menu interface and also have the md-menu
attribute. Additionally menu items are list items so they must also have the
md-list-item attribute.
Menu items can control a menu by selectively firing the close-menu and
deselect-items events.
This menu item will open a sub-menu that is slotted in the submenu slot.
Additionally, the containing menu must either have has-overflow or
positioning=fixed set to true in order to display the containing menu
properly.