A callback function used by the type system to initialize a new instance of a type.
This function initializes all instance members and allocates any resources required by it.
Initialization of a derived instance involves calling all its parent types instance initializers, so the class member of the instance is altered during its initialization to always point to the class that belongs to the type the current initializer was introduced for.
The extended members of @instance are guaranteed to have been filled with zeros before this function is called.
GObject::TypeInstance -> Nil