Change preferences sub-dialogs to be modal

This solves the issue where the parent dialog is closed and then
the child dialog is used.

This is however only a partial fix:

- Many other dialogs throughout the codebase do not currently have
  parent windows and need to be refactored.

- Not all window managers respect modal so users can still trigger
  bugs. We can be more defensive against this but it requires more
  refactoring.

Closes #2686

src/fe-gtk/chanlist.c

@@ -512,7 +512,7 @@ chanlist_save (GtkWidget * wid, server *serv)
 	GtkTreeModel *model = GET_MODEL (serv);
 
 	if (gtk_tree_model_get_iter_first (model, &iter))
-		gtkutil_file_req (_("Select an output filename"), chanlist_filereq_done,
+		gtkutil_file_req (NULL, _("Select an output filename"), chanlist_filereq_done,
 								serv, NULL, NULL, FRF_WRITE);
 }