addConstraint 
  Adds a constraint to the layout manager.
The property@Gtk.Constraint:source and property@Gtk.Constraint:target properties of constraint can be:
- set to - NULLto indicate that the constraint refers to the widget using- layout
- set to the class@Gtk.Widget using - layout
- set to a child of the class@Gtk.Widget using - layout
- set to a class@Gtk.ConstraintGuide that is part of - layout
The @layout acquires the ownership of @constraint after calling this function.
Parameters
constraint
a class@Gtk.Constraint