[IZPACK-989] ConfigurationInstallerListener - XML merge fixes and improvements

This is a task with subissues:
- IZPACK-990 Merge action properties do not work - always full XML merge assumed for each xpath	 Major - Major loss of function.
- IZPACK-991 ConfigurationInstallerListener - result of xml merge should be grouped by element name
- IZPACK-992 ConfigurationInstallerListener - Update to JDom 2, increase XML parser speed