I have to write a desktop application to edit data stored in a XML file. The format is defined by a XML schema file (.xsd). The format is quite complex.
Are there tools
There is an experimental tool, which does that, automatically create a desktop UI for handling data stored in XML. It's called KXForms. It comes as part of a suite of tools for handling XML data in various ways, like creating C++ code from an XML schema to represent the data natively in C++ and encapsulate parsing and writing.
This might not be the production-ready solution you are looking for, but it's a start and a source for inspiration. It's open source, so contributions are certainly welcome.