some renaming
create XtextItemResourceRepository singleton using the factory design pattern
return the expected interface instead of the implementation
create the XtextSitemapRepository singleton using the factory design pattern
simplify the ItemXtdex instance generation
simplify the SitemapXtdex instance generation
simplify the interface instance generations of the model module
simplify the interface instance generations of the model module using the factory pattern
format the code
simplify all the client-visible interfaces' implementations
reduce the useage of nested classes