This does the bulk of the work previously done by message_box()
proper, but takes a pointer to a result-reporting callback function
identical to the one we pass to create_config_box().
The modal version of message_box() still exists and is a small wrapper
on this function, running its own subsidiary gtk_main() loop which the
result callback terminates. But now I can start switching over
individual uses of message_box() to the non-modal version, and when
that's done, remove the modal function completely.
46ba9de2c2