How should I use GtkHeaderBar without having a duplicate title bar on older platforms?

[Dylan McCall <dylanmccall gmail com>]

For my GSoC project, I'm writing an application that uses
GtkHeaderBar. Using Gtk 3.8, with libgd as a handy drop-in replacement
for the new features in 3.9+, it looks like this:
http://ubuntuone.com/0cr9GKuwrbULXMmL3eExmq

As you can see, my application ends up with the title in two places,
and I'm not sure how best to deal with that. I imagine I can detect
whether I'm running in an environment that does client-side
decorations and react accordingly, but then I'm still uncertain how
the application is supposed to handle this. Is it better if the header
just doesn't have a title in this case, or should I do something like
System Settings 3.8 where the window titlebar has the primary title
and the header bar has what would be the secondary title? Is there an
API that can deal with this for me so I don't have to keep track of
these special cases myself?

By the way, I'm happy depending on just Gtk 3.9+, but as I understand
it we still have to deal with the possibility that the window may or
may not have a titlebar provided by the window manager.

Thanks for any help!

--
Dylan