ESC key will destroy the windows mentioned at issue 211.

This change implements generic routine gtkutil_destroy_on_esc ()
and adds invocations of it in the requisite window-handling places.

src/fe-gtk/urlgrab.c

@@ -198,6 +198,7 @@ url_opengui ()
 	urlgrabberwindow =
 		mg_create_generic_tab ("UrlGrabber", _(DISPLAY_NAME": URL Grabber"), FALSE,
 							 TRUE, url_closegui, NULL, 400, 256, &vbox, 0);
+	gtkutil_destroy_on_esc (urlgrabberwindow);
 	view = url_treeview_new (vbox);
 	g_object_set_data (G_OBJECT (urlgrabberwindow), "model",
 	                   gtk_tree_view_get_model (GTK_TREE_VIEW (view)));