event-editor/summary-section: Inherit AdwPreferencesGroup

This is a prerequisite to make GcalEventEditorSection require
inheriting AdwPreferencesGroup.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/781>

src/gui/event-editor/gcal-summary-section.blp

@@ -1,21 +1,13 @@
 using Gtk 4.0;
 using Adw 1;
 
-template $GcalSummarySection: Adw.Bin {
-  ListBox {
-    selection-mode: none;
+template $GcalSummarySection: Adw.PreferencesGroup {
+  Adw.EntryRow summary_entry {
+    title: _("Title");
+    changed => $on_summary_entry_text_changed_cb(template) not-swapped;
+  }
 
-    styles [
-      "boxed-list",
-    ]
-
-    Adw.EntryRow summary_entry {
-      title: _("Title");
-      changed => $on_summary_entry_text_changed_cb(template) not-swapped;
-    }
-
-    Adw.EntryRow location_entry {
-      title: _("Location");
-    }
+  Adw.EntryRow location_entry {
+    title: _("Location");
   }
 }