To change KDevelop's code please read the API documentation generated by doxygen at:

http://www.kdevelop.org/

or generate it yourself:

/path/to/kdelibs/doc/api/doxygen.sh --recurse --doxdatadir=/path/to/kdelibs/doc/common /path/to/kdevplatform  ( the generated documentation will be at kdevplatform-apidocs )

or read *.dox files in the doc/api/ directory.