GTK 2 has deprecated it and provided no replacement; a bug tracker
entry I found on the subject suggested that it was functionality that
didn't really belong in GTK, and glib ought to provide a replacement
instead, which would be a perfectly fine thing to suggest if they had
waited for glib to get round to doing so *before* throwing out a
function people were actually using. Sigh.
Anyway, it turns out that subsidiary invocations of gtk_main() don't
happen inside GTK as far as I can see, so all I need to do is to make
sure my own invocations of gtk_main() are followed by a cleanup
function which runs any quit functions that I've registered.
That was the last deprecated GTK function, so we now build cleanly
with -DGTK_DISABLE_DEPRECATED. (But, as mentioned a couple of commits
ago, we still don't build with -DGDK_DISABLE_DEPRECATED, because that
has migrating to Cairo drawing as a prerequisite.)
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