Fall back to use the floating references API in glib if there isn't a sinkfunc defined.

* tests/*: Add ref counting tests for floating objects
* gobject/gobjectmodule.c, gobject/pygobject.c: Fall back to g_object_ref_sink
  or g_object_ref if there isn't a sinkfunc defined. Make sure that
  pygobject_sink gets called only once per GObject instance.

https://bugzilla.gnome.org/show_bug.cgi?id=583909