girepository: Fix declaration of ‘find using interfaces’ methods

Fix the declaration and documentation of gi_object_info_find_method_using_interfaces() and gi_object_info_find_vfunc_using_interfaces(). The documentation was wrong when I wrote it: the value returned is the object or interface which declares the method or vfunc, not the one which implements it.

The returned declarer info may be a GIObjectInfo or a GIInterfaceInfo. Since those two types have no subtype relation between them, the return type has to be changed to GIBaseInfo. Using GIObjectInfo would have been fine in girepository-1.0 because all *Info types were aliases of each other — but since the move to GTypeInstance this is no longer true.

A unit test will be in the following commit.

Signed-off-by: Philip Withnall pwithnall@gnome.org

Fixes: #3246 (closed)

Closes #3246 (closed)