Issue: Generic Parent- / Child- list Updater

This is one of the more confusing aspects of developing a hierarchical system - getting a cached parent or child list for each node so that you don't have to query the database X + Y times to get a breadcrumb path and display a child-node directory. A smart Hierarchy DM should be able to do this with 1 or 2 queries, and be able to handle the deletion of entire sections of the structure.

Added and currently implemented for the Area DM. Having the generic system for this will save development time and package file space for the other hierarchical structures, like Books and Categories.