NodeModel.clearTree() now returns information about removed elements.

This is necessary to correctly populate a CLEAR_TREE event for hierarchical
configurations. InMemoryNodeModel here returns a list with QueryResult objects
for the nodes or attributes selected by the key passed to clearTree().

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1588818 13f79535-47bb-0310-9956-ffa450edef68