gcal-event-editor: Add mnemonic for "Delete Event"

This lets keyboard-centric power users delete events in 1 second.
It can be useful in rare situations when you need to delete many events.

Fixes: https://gitlab.gnome.org/GNOME/gnome-calendar/-/issues/1313
Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/628>

src/gui/event-editor/gcal-event-editor-dialog.blp

@@ -78,7 +78,8 @@ template $GcalEventEditorDialog: Adw.Dialog {
         Adw.PreferencesGroup delete_group {
           Adw.ButtonRow {
             activated => $on_delete_row_activated_cb(template) not-swapped;
-            title: _("Delete Event");
+            title: _("_Delete Event");
+            use-underline: true;
 
             styles [
               "destructive-action",