GTK 3: tweak margin below the gtkask drawing areas.

A side effect of commit 1f9df706b seems to have been to squash those
areas right up against the bottom of the dialog box, which is ugly. I
don't fully understand why it only happens to those drawing areas and
not to buttons placed in the fake 'action area' by other dialogs, but
anyway, adding an explicit margin-bottom attribute seems to solve it.

unix/gtkask.c

@@ -375,6 +375,12 @@ static const char *gtk_askpass_setup(struct askpass_ctx *ctx,
                          G_CALLBACK(expose_area),
                          &ctx->drawingareas[i]);
 #endif
+
+#if GTK_CHECK_VERSION(3,0,0)
+        g_object_set(G_OBJECT(ctx->drawingareas[i].area),
+                     "margin-bottom", 8, (const char *)NULL);
+#endif
+
         gtk_widget_show(ctx->drawingareas[i].area);
     }
     ctx->active_area = rand() % N_DRAWING_AREAS;