[Doxygen-develop] Wish: have an unimplemented attribute

[Paul D. <duf...@gm...>]

For a big project, we sometime begin by writing stub functions just
returning a no error code. And it is sometimes hard to know what have
been implemented or not without going all the sources.

Being able to add an unimplemented attribute, would be nice.
Moreover if the list of functions (or members, etc) that still need to
be implemented would be listed, perhaps inside the todo page.