Prevent orphaned menu items

The ajax-based menu manager isn’t 100% reliable. I was making some changes to the menu and ended up loosing a parent entry for no apparent reason. And because all the child-entries were set for “one-time use only”, I lost the ability to add forums, gallery and downloads. 

It would be good to add some “self-healing” capabilities. For example, when the ACP menu manager is opened, check each entry and if necessary, reset them if they link to a non-existing parent.