gdbus-codegen supports file descriptors as the following anntation and GUnixFDList.

<annotation name = "org.gtk.GDBus.C.UnixFD" value = "true" />

It would be nice if gdbus-codegen-glibmm also supports file descriptors with the same annotation and Gio::UnixFDList.

I'm using glibmm's low API because the generator doesn't support it right now, so the code is unnatural.