Implement chaining of interface methods.

GObject interface methods are very similar to virtual methods.  A class
may wish to override some or all of the interface methods implemented
in a parent class.  When doing so, it can be useful to call the parent
class implementation to offload some of the work.

For ordinary virtual methods, using the "override" keyword causes a
PARENT_HANDLER macro to be defined which can call up to the parent
class implementation.  Let's do exactly the same thing for interface
methods, and expose the parent interface structures (in case the user
wishes to call up to different methods).