New takes a widget, rather than a menu model, so that I can potentially style the menu.

AutoCloserWidget is used to detect if a given menu is open when a widget responds to user input.

NavWrapperWidget is used to detect if a given menu is open when a widget responds to user input.

SiteWidget is a zero-width widget which acts as the coordinates at which a submenu will open.

Widget overlays one widget on top of another.

Return false if it was already open.