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/theme/style.css

@@ -491,3 +491,10 @@ syncindicator {
 .blurred {
   filter: blur(6px);
 }
+
+/* Participants section of the event editor dialog */
+.participants-count-label {
+   border-radius: 12px;
+   background-color: color-mix(in srgb, currentColor 10%, transparent);
+   padding: 6px 9px;
+}