Done. If a user now clicks on a row in the Workbench sidebar then the current project is closed, the new one opened and **after that** ```document_open_file()``` is called.
@codebrainz, @elextr: the ```ProjectPrefs``` structure is actually not exposed to the plugin API therefore I agree to elextr and would like to keep the setter and getter function. But I renamed the item ```project_session``` to ```use_project_session_files```, renamed the function names to and extended the functions doc comment a bit. I hope that's sufficient.