Add tracking hooks

Here is a small patch that adds some tracking hooks (just insert and delete) so that sub-modules can be created that rely on MNE to watch the menu forms.

These hooks are fired when menu items are created or deleted. The insert hook is not great, since it also fires on update, technically, so your module would need to run a query to see if it needs to fire. This is mainly used for other modules that want to know if menu items have been deleted.