Provided an implementation for addPropertyDirect().

This method is needed by some concrete sub classes for adding properties
verbatim - without doing list parsing.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1588680 13f79535-47bb-0310-9956-ffa450edef68
Reworked XMLPropertyListConfiguration to be compatible with immutable nodes.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1588681 13f79535-47bb-0310-9956-ffa450edef68