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/gui/event-editor/event-editor.gresource.xml

@@ -13,5 +13,8 @@
     <file compressed="true">gcal-summary-section.ui</file>
     <file compressed="true">gcal-time-zone-dialog.ui</file>
     <file compressed="true">gcal-time-zone-dialog-row.ui</file>
+    <file compressed="true">gcal-attendees-section.ui</file>
+    <file compressed="true">gcal-attendee-row.ui</file>
+    <file compressed="true">gcal-attendee-details-page.ui</file>
   </gresource>
 </gresources>