event-editor: Hide Cancel & Delete when read-only

Read-only events show a "Done" button instead of the "Save" button.
The "Done" button does the same thing as "Cancel",
so the "Cancel" button is then redundant.

Likewise, it is impossible to delete events from read-only calendars,
as stated by the banner message in that dialog. In that case,
there's no reason to show a big "Delete" button that you can't click on.

Fixes https://gitlab.gnome.org/GNOME/gnome-calendar/-/issues/1168

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

@@ -78,7 +78,6 @@ template $GcalEventEditorDialog: Adw.Dialog {
         Adw.PreferencesGroup delete_group {
           Adw.ButtonRow {
             activated => $on_delete_row_activated_cb(template) not-swapped;
-            sensitive: bind template.writable no-sync-create;
             title: _("Delete Event");
 
             styles [

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

@@ -702,8 +702,10 @@ gcal_event_editor_dialog_set_event (GcalEventEditorDialog *self,
 
   set_writable (self, !gcal_calendar_is_read_only (calendar));
 
+  gtk_widget_set_visible (self->cancel_button, self->writable);
+
   self->event_is_new = new_event;
-  gtk_widget_set_visible (self->delete_group, !new_event);
+  gtk_widget_set_visible (self->delete_group, !new_event && self->writable);
 
   if (!self->writable)
     gtk_widget_grab_focus (self->done_button);