Adding hint for plantuml file format
A very good idea to integrate a model editor in the QtCreator. just an hint to the fileformat. Have investigated plantuml? (http://plantuml.com/) I use this heavenly in code development and documentation and has many plugins e.g. for word, sphinx etc. So the model could be easily integrated into other tools without explicitly generate exporters/importers for it.
If you separate content (model definition) and gui parts (positions, colors) etc the model could be used for other purposes also.
Just a suggestion