Well I'm already doing that. I don't want Geany to decide which project to open. That is done by the Workbench plugin. I just need access to the functions for opening and closing of a project. And a way to shortly change the global config setting "Use project-based session files". That's all. I just wanted to explain what I need it for in the Workbench plugin. If you look at this PR there are not much changes in it. It does not include any decision making for project opening/switching - I just wanted to explain the context.
But as answered to @codebrainz I am willing to remove the setter and getter function and expose the struct members of ProjectPrefs as part of the API if that is preferred.