U++

I think CodeEditor should also highlighting XML Schema files (.xsd extension):

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Changes:

ide/Common - ComDlg.cpp - line 30:

if(ext == ".xml" || ext == ".xsd")

ide/Common - ComDlg.cpp - line 114:

String mask = "*.cpp *.h *.hpp *.c *.C *.cc *.cxx *.icpp *.lay *.iml *.lng *.sch *.usc *.rc *.brc *.upt *.html *.js *.css *.witz *.json *.xml *.xsd *.qtf";

CodeEditor - CRegister.iccp

EditorSyntax::Register("xml", callback1(CreateTagSyntax, false), "*.xml *.xsd", "XML (.xml)");

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P.S.
XML Schema has got exactly the same highlight as XML. So we don't need to change any CodeEditor core components.

P.S. 2
More about XML schema you can find on following websites:
http://en.wikipedia.org/wiki/Xml_schema
http://www.w3schools.com/Schema/