[DEPRECATE] Listings only have titles

[Rob Beezer]

Another step in the long journey to figure out "figure-like". Courtesy of
Andrew Scholer.

#listing now have a #title and no #caption. That means the only figure-like to
have a #caption is #figure itself.

You will get a deprecation warning, but if you have a #listing with *only* a
#caption, we will treat it as if it were a #title. If you have both a #caption
and a #title, you will need to determine what to do with your #caption.

This code has seen a lot of changes over the years, so holler if anything seems
amiss.

Thanks, Andrew.

Rob