Reworked PatternSubtreeConfigurationWrapper.

The class no longer extends AbstractHierarchicalFileConfiguration. Rather, it
wraps a plain hierarchical configuration which also has to implement the
FileBased interface.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1463694 13f79535-47bb-0310-9956-ffa450edef68