The config box setup code wants a lot of very narrow edit boxes, so it
decreases their minimum width from the GTK default of 150 pixels. In
GTK 3, we have to do this using gtk_entry_set_width_chars() rather
than gtk_widget_set_size_request() as we were previously using, or it
won't work.
This change by itself seems to restore the GTK3 config box to a
sensible layout, in spite of the fact that my Columns layout class is
still doing all its size allocation the GTK2 way, with no attention
paid to the shiny new GTK3 height-for-width system.
5e738877be