A GtkTearoffMenuItem is a dedicated type of
GtkMenuItem that can be prepended to a menu
in order to give the user the option of tearing off the popped-up menu
from its parent object (usually a menu item in a
GtkMenuBar), and keeping it within an independant,
non-resizeable GtkWindow of its own.
The widget appears as a dotted line across the top of the menu when
the menu is attached to a parent object, or as a dotted line with a
left-pointing arrow where the menu is detached. Clicking on the dotted
line detaches an attached menu, and clicking on the arrow of a detached
menu re-attaches it to the parent.
