UX/UI designed by Philipp Sauberzweig (@PhilippSauberz). The event editor dialog is now a NavigationView and the participants details open in a separate page. Group attendees/participants in AdwPreferencesGroups and bind groups' ListBox widgets to backing filter models. These are just "views" into a single GListStore containing all the event attendees. Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/671>