Add functionality to allow modification of the navigation menu via a hook

Other modules (like civischool / civiengage) need to be able to add navigation menu items to the civicrm tool bar

ideally we cn do this via a nav menu hook (during a build/rebuild phase) or via hook_enable

We should be able to:

• create a new menu item
• add children to this item
• get the list of menu items for any parent