logic for preventing duplicate definition of top level elements are added
relaxed xsd from sequence to choice and added import element
initial config import implementation with duplicate and cycle detection.
Xml Configuration Composition documentation added
XmlElements and XmlConfigPreProcessor classes are extracted from XmlConfigBuilder. Some typos fixed
XMLConfigPreProcessorTest extracted from XMLConfigBuilderTest and some minor refactoring made to XmlConfigPreProcessor to make methods shorter
Merge pull request #3893 from eminn/XMLConfigImport

Xml Config Import