introspect: need a better app-id
Currently, when calling GetWindows, I get desktop file names as app-id. That is pretty useless for me in the portals, where I need a flatpak app-id. It would be best to send me whatever you have, clearly marked: desktop-filename should be separate from app-id (you can get the app-id for flatpaks from the desktop file, with the X-Flatpak key).