Clean up container title functions

* Add and use lenient_strcat and lenient_strncat functions
* Rename `concatenate_child_titles` function as that's no longer what it
does
* Rename `container_notify_child_title_changed` because we only need to
notify that the tree structure has changed, not titles
* Don't notify parents when a child changes its title
* Update ancestor titles when changing a container's layout
	* Eg. create nested tabs and change the inner container to stacking
* No need to store tree presentation in both container->name and
formatted_title

sway/commands/layout.c

@@ -52,6 +52,7 @@ struct cmd_results *cmd_layout(int argc, char **argv) {
 		}
 	}
 
+	container_notify_subtree_changed(parent);
 	arrange_children_of(parent);
 
 	return cmd_results_new(CMD_SUCCESS, NULL, NULL);