PropertyExpression 
  Creates an expression that looks up a property.
The object to use is found by evaluating the expression, or using the this argument when expression is NULL.
If the resulting object conforms to this_type, its property named property_name will be queried. Otherwise, this expression's evaluation will fail.
The given this_type must have a property with property_name.
Return
a new GtkExpression
Parameters
The type to expect for the this type
Expression to evaluate to get the object to query or NULL to query the this object
name of the property
Creates an expression that looks up a property.
The object to use is found by evaluating the expression, or using the this argument when expression is NULL.
If the resulting object conforms to this_type, its property specified by pspec will be queried. Otherwise, this expression's evaluation will fail.
Return
a new GtkExpression
Parameters
Expression to evaluate to get the object to query or NULL to query the this object
the GParamSpec for the property to query