A menu item that can be "torn off" into a floating window.
A GtkTearoffMenuItem is a special GtkMenuItem which is used to tear off
and reattach its menu.
When its menu is shown normally, the GtkTearoffMenuItem is drawn as a
dotted line indicating that the menu can be torn off. Activating it causes
its menu to be torn off and displayed in its own window as a tearoff menu.
When its menu is shown as a tearoff menu, the GtkTearoffMenuItem is drawn
as a dotted line which has a left pointing arrow graphic indicating that
the tearoff menu can be reattached. Activating it will erase the tearoff
Creates a new tearoff menu item.