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/gcal-attendee-details-page.h

@@ -0,0 +1,36 @@
+/* gcal-attendee-details-page.c
+ *
+ * Copyright (C) 2025 Alen Galinec <mind.on.warp@gmail.com>
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef GCAL_ATTENDEE_DETAILS_PAGE_H
+#define GCAL_ATTENDEE_DETAILS_PAGE_H
+
+#include <adwaita.h>
+
+#include "gcal-event.h"
+
+G_BEGIN_DECLS
+
+#define GCAL_TYPE_ATTENDEE_DETAILS_PAGE (gcal_attendee_details_page_get_type ())
+G_DECLARE_FINAL_TYPE (GcalAttendeeDetailsPage, gcal_attendee_details_page, GCAL, ATTENDEE_DETAILS_PAGE, AdwNavigationPage)
+
+void                 gcal_attendee_details_page_set_event          (GcalAttendeeDetailsPage *self,
+                                                                    GcalEvent               *event);
+
+G_END_DECLS
+
+#endif