GTK 3 cleanup: stop using GtkDialog for config boxes.

GTK 3 cleanup: stop using GtkDialog for config boxes.

It's becoming annoying to keep working within the increasing
restrictions on GtkDialog, in particular the fact that not only do we
have to let it have complete control of the button area, but also it's
not clear whether we can intercept a press of the 'OK' button and
display an error message rather than ending the dialog.

So, as I did in PuTTY, I'm resorting to using an ordinary GtkWindow
with controls I laid out myself.