Helper function for constructing Initable object.
This is similar to @new but also initializes the object and returns null, setting an error on failure.
optional Cancellable object, null to ignore.
the value if the first property, followed by and other property value pairs, and ended by null.
a Error location to store the error occurring, or null to ignore.
the name of the first property, or null if no properties
a newly allocated Object, or null on error