Move child info retrieval into generic function
Move child info retrieval into generic function

Add a generic function for bindings which return a single child info.
This trivializes binding methods like PyGIObjectInfo.get_parent and
fixes leaks in PyGIObjectInfo.get_class_struct and
PyGIVFuncInfo.get_invoker.

https://bugzilla.gnome.org/show_bug.cgi?id=709008

davibe/pygobject,davibe/pygobject,davibe/pygobject,sfeltman/pygobject,pexip/pygobject,davibe/pygobject,pexip/pygobject,sfeltman/pygobject,GNOME/pygobject,GNOME/pygobject,sfeltman/pygobject,GNOME/pygobject,pexip/pygobject