event-editor: Add attendees section and details page

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>

src/utils/gcal-utils.h

@@ -136,4 +136,5 @@ SoupSession *        gcal_create_soup_session                    (void);
 
 GListModel *         gcal_create_writable_calendars_model        (GcalManager        *manager);
 
+const gchar *        gcal_get_email_from_mailto_uri              (const gchar        *mailto_uri);
 #endif /* __GCAL_UTILS_H__ */