a function to retrieve the GtkSizeRequestMode of the widget using the layout; the default request mode is %GTK_SIZE_REQUEST_CONSTANT_SIZE
a function to measure the widget using the layout manager
a function to allocate the children of the widget using the layout manager
the newly created GtkCustomLayout
ConstructionException GTK+ fails to create the object.
Creates a new legacy layout manager.
Legacy layout managers map to the old GtkWidget size negotiation virtual functions, and are meant to be used during the transition from layout containers to layout manager delegates.