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>
mindonwarp