Adapted TrackedNodeModel to change in NodeModel interface.

The clearTree() method now returns the result of clearTree() of the underlying
model.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1588819 13f79535-47bb-0310-9956-ffa450edef68
Reworked the return type of clearTreeInternal().

The method can now return an arbitrary object which is used as value when
creating a configuration change event. It is up to the NodeModel in use how
this value is constructed.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1588820 13f79535-47bb-0310-9956-ffa450edef68
Adapted return type of clearTreeInternal().

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1588821 13f79535-47bb-0310-9956-ffa450edef68