Added trackChildNodeWithCreation() method to InMemoryNodeModel.

This method allows tracking a child node of a selected node. If the child does
not exist, it is created dynamically.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1580594 13f79535-47bb-0310-9956-ffa450edef68
Some cleanup of methods related to sub configurations.

Removed unused methods. Made a method protected which is needed by sub classes.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1580596 13f79535-47bb-0310-9956-ffa450edef68